If you trust someone enough and feel they have earned the right to hold an item in the first place, than you should just give it to them. If Kirin's Osode was r/e in the first place, like most of the other sky armor, than this wouldn't even be an issue.
I've always felt that the idea of "shared property" was silly and unenforceable and unfair. Just because some of the best items for certain jobs happens to not have that ex tag, doesn't mean someone should have to give it up. I mean, in Dynamis, you don't ask people to return AF2 when they leave the LS. It's something they "earned". It should be the same way with sky items.
If you feel someone in your LS has earned the right to use armor like that based on their involvement, then you shoould just let them keep it. If you're afraid people will take advantage of this, then don't give them the item in the first place.
My own LS uses a point system. A person can't take an osode or a tunic until they have participated enough to earn that item. After which, it is their's.
Rather then enlist ideas to get this person to return the osode (he either will or he won't...and at this point I think it's more likely he won't) you might want to enlist ideas from ppl here for how they handle drops of this sort in their own LS's. Keep the problem from happening again.
Nicholas
Gilgamesh - Bastok Guardians