I looked around in the WoW API and at first glance this doesn't seem to be possible, but I just want to know if there's something I may have missed.
My original idea was to create a dewdrop menu when you alt-click/alt-rightclick an item that would list all possible enchants to that item, which you could then click on to cast that enchant. The trouble lies in that there doesn't seem to be any way to cast enchants through an addon.
Dewdrop supports secure actions, so it may actually be possible to do this. Unfortunately, WoWWiki doesn't have any documentation on the function you'd need (DoCraft), so you might have to spend a good chunk of time playing around with things to get it working.
I'd love to be able to mouseover someone's equipment and have the enchants/armor kits I can do show up, then be able to alt-click the enchant to do it when the trade window pops up.
I know there are several mods that can compare equipment and expand out next to the slot, showing the other stuff I could equip, there's gotta be a way to show up a simple clickable list of enchants too :p