When this has been asked in the past, the official response has usually been something along the lines of "they work slightly differently to costumes but we're looking into it". May I ask if there has been any update on this and whether it's still a possibility? And if so, would they be dyeable?
One of the most workable suggestions from the community for this that I can remember is adding in an outfit version of each disguise as a quest reward in the related questline for example.
I know one argument against is that there's no money in it etc, but it would show a great deal of goodwill towards the playerbase, and it is something that's been on our minds since the original introduction of the crown store and collections window.
Allow cross-platform transfers and merges