I regret to report that both the potion of illusion and mage glyph of illusion have been coded to drop the buffs several seconds after logging in with that model stored. Presumably this would apply to the skymirror as well. The things Blizzard prioritizes for fixes over other far more relevant things will never cease to amaze me.
You can however still get the tree disguise kit model from the quest tree's company in the Draenai noob zone provided you get the model from somebody else who is using the kit. Best model I have to change into right now without a rogue to pickpocket something.