Yeah, it would definitely be a hobby device, for me. Rainy day fun project. I sold my MTII long ago, and just have the R-44 for digi in when needed. This one looks a little nicer, but looks to be digi out only:
http://www.hifiberry.com/But, alas, I realized mine are Rev 1, so unsupported for these daughter boards. I could do something with USB, like you mentioned. I'll probably just leave the RPi's on the TVs to stream movies from the basement PC for the kids. They do a pretty good job at that (even if one of them never gets used), and the kids could care less what that thing is that makes the magic happen.
--Michael