They could make it possible (albeit difficult in some cases) simply by removing the "UNIQUE" tag from gear. Want a robe that has one look when wearing it as a White Mage, another when you wear it as a Scholar, and a third look when wearing it as an Astrologian? Just collect three of the same robe and glamour them differently. That version wouldn't require any change to how glamours work, would still allow gear sharing (by players who don't care about the different appearances), and would allow different appearances by players who are willing to get multiple pieces. The only thing holding it back is pieces that we're not allowed to get multiple versions of.