They should all be bitUSD.
complete with cross blockchain trading.
You still need price discovery mechanism. IE an exchange. So what you guys really want is a decentralized exchange.
If that's the way it has to be done, then sure. But it should appear as though you can send bitUSD from one of the music frontends, to a BitShares X wallet, and back. A silent, automatic, decentralized exchange that runs in the background could facilitate this, but it would be cool if bitUSD could be supported by multiple blockchains. Then the bitshares project would be like a network of networks.