This a theory of mine. The first (obvious) evidence is that the masks on solstheim look much different than the normal ones (all tentacle like). The second is that the masks are daedric and benefit from the perk. The priests all also have shrines to herma-mora. Dukaans name means dishonor,and it's possible that he was involved in miraaks uprising. My last evidence is that the other named dragon priest vahlok the jailer who was possibly the only loyal priest had no mask.

How plausible is this theory? And is there any other evidence to improve upon/ disprove this theory?

Seems reasonable, though Ahzidel and the others could have made their own through Mora's own secrets.

Yes and the tentacular design of the masks is definitely a giveaway. Although all the other dragon priest masks are all ascribed to the daedric smithing perk.

Though are linked to being handed down by the dragons themselves. Going to have to go with them being created by quasi-divine beings and thus require knowledge in dealing with a similar substance in order to improve on them then actual Daedric involvement. If only for the sake of not giving the Daedra a monopoly on everything seemingly cool or semi-relevant in the universe if nothing else. Some dragon's are adept enchanters, if Twin Secrets is to be taken at face value.

Exactly. I think it's a game play thing. There are heavy masks as well as light ones, but the dragon smithing perk is on the heavy armor side, while both the heavy and light armor perk lines lead to the daedric smithing perk.

BTW the coffin of Zahkriisos stands before a word wall for Miraak. So I think it's safe to guess he served Miraak. I don't know about Ahzidal though. He seems to be much cooler than Miraak.

EDIT: Oops made a mistake. The dragon smithing perk can be reached from both light and heavy sides. The daedric smithing perk is on the heavy side. Haven't played Skyrim for so long. And now I really think it a mistake to assign the daedric smithing perk to dragon priest masks.