Can you add the ability to fetch the BTC address for the BTS Public Key under an accounts Active Key History and then provide a JSON RPC API that will allow people to lookup Bitcoin addresses for people that have registered accounts on BTS? Perhaps have an option to return the address in several of the top crypto currencies.
This way people can use our ID system for all crypto platforms and easily share it with people.
This is better accomplished by writing a standard spec for users to specify addresses for other cryptos in public data or object graph.
Importing the same private key into N different cryptocoin clients means that
all of the different cryptos owned by that key will be lost if
any of the clients that has that key is compromised.
My worry is that some of the more obscure altcoins may not have enough eyes on their source to avoid key stealing back doors. And the more we encourage behaviors that require people to re-use the same keys in multiple clients, the more potentially lucrative the attack vector would become.
You might be able to use Diffie-Hellman to take a user's public key K and the name of the altcoin, say N = "DOGE", and determine some new pubkey K' = f(K, N), then K' is your Doge key. Clearly you can get the private key for K' if you have the private key for K -- that's kind of the point of this whole construction. But if you can "go backwards", i.e. get the private key for K from the private key for K', then it defeats the purpose (compromising your Doge private key gives the attacker enough to derive your BTS private key). I'd have to study ECDH in detail for a while to figure out if "going backwards" is possible or not.