We can accomplish this with a pretty straight forward Smart Contract on Bitshares I believe.
The process would basically involve creating an EBA (UIA) that interfaces with a hosted wallet of said coin.
Similar to how blocktrades works, a deposit address associated with the EBA would be generated.
When the deposit hits the address a transaction is sent via the smartcontract to issue the EBA to the associated Bitshares account.
When the user wants to withdraw, they would perhaps send the amount they want to withdraw to a given bitshares address with a certain memo to specify the target address. The EBA is removed from their account and the coin is transferred to where ever they wanted it to go.
This would require maintaining a very secure environment.. but it's been done before.
This would effectively transfer coins in and out of bitshares in their full value.
It's doable.. I bet blocktrades is already like 90% of the way there.