How about a system where (just for high level unique or difficult to obtain items) they added a system that would essentially let you get a second untradeable/unsellable/un-desynth-able copy of an item much more cheaply/easily once you've obtained the first one. That way, even on items that could (and therefore normally would) be shared between classes, you could instead just get multiples of them, and glamour one for each look you want.

Advantages:
1) It keeps the glamour system itself pretty much unchanged, which I'm assuming would help developers (and whatever system limitation concerns led to the initial development of this system rather than cosmetic slots).
2) It provides a lot of flexibility. You could set different glamours by job, but could also set up different glamoured sets to suit your mood if you want.

Disadvantages:
1) May require more Armoury Chest space.