I can see more than a few problems with trying to port MC to Linux or OS-X. Don't get me wrong -- I would love it, but things like the codecs would prove interesting....
Rather, how about a small step to start things off.
1. MC already has the library server.
2. The library server can already transcode to common formats.
Build a client version of MC for other OS's. It would use the library server, which would have transcoded as needed, avoided the codec issues, drivers etc. Plus, it allows JRiver to make a series of companion products, which I'd at least buy:
1. UPnP client
Nero claims to have one, but I have it, and JRiver could easily do better. The client would be very lightweight as it doesn't need to carry most of MC. It would require MC11 on the backend.
2. Web-based MC player
For cases where MC11 can't be installed, this is stripped version of MC in library client mode. Basically, it's a player only. TwonkyVision is trying to do this right now.