I've not played Bloodmoon, so perhaps this is explained there and I've overlooked the information.

But is there any canonical explanation as to why some individuals have greater control over their transformation than others?

I'm imagining that perhaps those that are inherently loyal to hircine and/or are natural hunters are given the leeway to use the blessing as they see fit since they perform a satisfying hunt in whatever form they take, whereas the more cowardly and unwilling are forced to become hunters against their wishes... but that's entirely guesswork based on the fact, the more nondescript the character in the lore, the less control they display over their transformations, Vs the dragonborn or the companions who display a great deal of control.

This could of course be explained other ways (The companions undergo a blood ritual at a place of ancient power as opposed to infection, or even just the result of red-shirt syndrome; the more disposable the character, the easier it is to make them an uncontrolled transformer)

What sort've control did the player character display over his lycanthropy in bloodmoon?

In Bloodmoon it was every night I think. I never got to be a Werewolf in bloodmoon but when Skyrim was announced I read the whole GoTY physical guide.

Depends on how well willpowered one is. Plus, in Daggerfall, one could transform at will, though certain times the PC was forced to transform.

In Bloodmoon it was a nightly transformation. But we can assume it's that way because the Bloodmoon hunt was hosted by Hircine speficially for werewolves. And in lore, werewolves transform twice a month, or more(In Daggerfall they can transform at will, but still succumb to lunar transformations.) So I doubt Hircine would want his Hounds to morph every month. It would make sense to have them transform every night. And their werewolf powers are increased too under the Bloodmoon. In Daggerfall, regular werewolves have enhanced strength, speed and etc.

The only way in lore for a werewolf to have their transformation under control was using Hircine's ring. It controls the werewolf's transformation and bloodlust. In Daggerfall and Bloodmoon. In Skyrim, it works differently for the player. I think all werewolves, or most in general, have forced lunar transformations. The Circle within the Companions are an exception because we know they made a pact with the Glenmoril witches to become more powerful. We don't know what this power would be, but we can assume it's transform at will and not suffer from too much bloodlust. After all, the werewolf thing was in the companions for a few centuries, and only Kodlak's predecessor made the pact with the witches(With a "cost"). Sinding had Hircine's ring to try and control his transformations, meaning he still succumbed to lunar transformations and did not have that much control at all. He even mentions that he is a werewolf that transforms under the influence of the moons.

Since we can only become a werewolf through the companions, we know that lunar transformations are still canon. The Circle, so far, are the only werewolves that can control it. But we heard of one instance in the ingame book "Physicalities of werewolves." We shouldn't look too much into it because Bethesda might of added "Transform at will" for the sake of gameplay being easier. Willpower might have a place into it as well. A good number of werewolves go insane and feral, as Aela mentions. They succumb to the beast within. In Bloodmoon you could find insane wanderers and ravening lunatics which are all werewolf NPC's in their human forms. So they became crazy individuals who live in the wild. Then again, some can accept their Lycanthropy and enjoy the thrill of the hunt and still have a sense of "humanity" in them. But so far in Daggerfall, Bloodmoon and Skyrim, Lycanthropy is known to befuddle a person's mind to where the line between Man and Wolf is blurred.

Not trying to rain on Werewolf&Vampire's parade here, given the name they obviously feel they know a great deal
about lycanthropes, but after reading it, it is mostly based on game mechanics and not Lore encounters and studies
on lycanthropes (meaning all the varietes not just werewolves)

In the companions Alea makes a comment after your first transformation about she wasn't sure if you would come back
this is complete canon as when someone is infected with lycanthrope there is several unknown variables that are unique
per person. If they can't handle the transformation physically and mentally they will go crazy and become a wild man
like we seen in Bloodmoon, mostly naked people just wandering the woods unable to reenter society and just waiting
for the next change. Some can renenter society but have no control over their transformation and still lose themselves
during it as we've seen in stories about werewolves, where they were once decent people but when they change they
are just blood lusting monsters who will kill anyone around them. Sinding shows remarkable self control over the disease
by changing at will and showing logical thought while transformed. The cursed ring is what messed him up as it was
cursed to work the opposite way as intended by Hircine so it would force him to change randomly, so here we have
a lycanthropic werewolf with remarkable control over it, but wasn't satisfied so he stole Hircine's ring and Hircine cursed
it so he would be forced to change at completely random times and someone just happened to get hurt because of it.

The companions are in no way the Only werewolves who can control it, The old man picks people he thinks can
handle the transformation and if they can't, then they go feral so its really a toss of the coin if you don't got utterly
mad from the transformation or not, as there is quite a few recorded incidents of people other then the companions
that display some control or exceptional control over it, without Hircine's ring. Who knows that Breton from the
stories might of been a powerful mage.. wait he was a Breton so of course he was probably a powerful mage
Perhaps he used magic to assist him or even just his specific knowledge gave him greater insight into how
the transformation works and that allowed him to force it to come and go, which eventually the strain on his
body killed him.

And there is also stories of individuals with remarkable physical control over the transformations and even mental control
while changed as we seen in a few books from scholars who studied werewolves and a particular Breton man
who displayed a great affinity to be able to change back and forth. Wether this was from an exception constitution or
willpower, or possibly a boon from Hiricine for having participated in a ritualistic hunt in Hircine's name who knows.

In Bloodmoon the nightly change is unique to only that event because Hircine, while the player character performed duties
for Hircine they were rewarded with boons, stronger, faster, sharper claws, so this might be one way of obtaining abilities
that are unique from the average werewolf. From the best I can tell after reading almost every book I can find in -game about
the various types of lycanthropes some still being considered a myth like the wereshark, is that it perceived as a disease
even by those that live with it and remain the same person they were much the same way as people in our world would
deal with a death sentence disease. Some people just give up, some people learn to life with it, and some people embrace
it and use it, while they might use it for awareness propaganda and fund raising, a werewolf who embraces the disease
would probably have much more control over it then someone who fights it all the time.

All lycanthropes change under the moon regardless of how much control they have over the disease, if they go completely
feral or retain even a shred of self control is purely assumption on our part as we have not been in that position, outside
gameplay mechanics of Bloodmoon which can't be taken as lore.

The only recorded lore that would suggests a difference between werewolf species is in Skyrim, were Nord's boast that the
lycanthropes, werewolves and werebears are bigger and stronger then other areas of Tamriel but there has been no
proof of this and is mostly dismissed as Nordic Boasting in line with all their tales and myths were everything has to be
bigger and more glorious then it really was.

I never actually went against those things. I said lunar transformations are canon and there was that instance of a breton in the "Physicalities of werewolves" book. I even said about the Bloodmoon NPC's and the apparent willpower thing. But in a way, you are correct. I overlooked Aela's comment on the first transformation which is always "The most intense." as she would say. But, as you have mentioned and as I have mentioned before, werewolves and Lycanthropes in general can suffer from mental instabilities.