I do not currently have a table, I'm sad to say; however, I would happily allow it in one of my games, as without Medium armor proficiency it still has a decent ACP. I think it would be fun to make obtaining one a mini-quest; it would either be a legendary item that mages have sought for years or it would be up to the player to go through the (complicated) process of getting all the people and parts necessary.
EDIT:
I'm looking at it in the Draconomicon now (page 117). It looks like it has similar properties to mithral, but can be applied to hide armor, whereas mithral cannot. I'll see if it changes anything.