Ive asked them several times in several places, no answer. I have low expectations to Zenimax, mostly because of the whole lag issue that they refuse to take respondsibility for, but even then, I would be somewhat surprised if they didn't make costumes dyable.
If they DON'T, then only people who like the default appearence of the costumes will purchase them.
If they DO, people will buy them if they like the default AND if they have an idea of a cool set of dyes on the armor.
Basically, they will earn more money if they do. One thing is for sure though, I personally might not buy ANYMORE costumes unless they become dyable. It's a feature I think should have been there from the beginning, and why they didn't do that is BEYOND me.
But let's hope so. Such feature would add a lot more customization to the game which is something I (nearly) ALWAYS welcome.
Might be a tad off-topic but I also would like to see them usable in combination with guild tabbards, as the current system where you can't use them together is a deal breaker for me personally.