I just got done helping Septimus Signus and had that looooong conversation with Hermaeus Mora.

I got to thinking afterward if one decides to become his disciple, does that change the Dragonborn DLC at all? I highly doubt it, but thought I'd ask and see if anyone had tried it out

Not too much, but it does have a few characters react to having read the Oghma Infinium. Including the obvious one, Mora himself. It changes how he refers to you during the quest.

thw dialogue in DB's MQ changes slightly

Interesting. It's been a long time since I played through Dragonborn, but isn't that "HM" trying to get you as a disciple, too? Or am I mixing up Apocrypha with the Main Quest?

I can definitely see the "final decision" being easier to make if one already followed HM

He is. Though I'd argue that it might not go as far as Miraak, even though many suspect it will, since if that was the case, then they would have probably went to similar lengths as the Shivering Isles. Although not in the same way, but still essentially being influenced a lot more by Mora. Although, then again, there are those powers you can obtain as you go through the books, but I'd think it would need to be much more substantial. Though they probably leave it up to player choice for a reason, and have those lines to refuse or accept, to go along with that reason. Although you still end up serving his interests, since not only is the knowledge of Bend Will unobtainable elsewhere(although some would vastly prefer it was), it is in your interest to stop Miraak.

Though playing someone who is utterly devoted to Mora, would indeed make the choice easy. Especially if they don't put much weight into what he wants for the third word of Bend Will. Which is where most have a problem. Though it's their character, and I understand why.

Well, what else do you expect from the guy whose spheres include fate?