I'd also like to figure this out, but I need my 360 repaired and MS isn't being too helpful.
I believe you can use Windows Media Player, so maybe you could try running that through emulation or through Wine, using samba to share? I'm not sure if the 360 is "tricked" by Samba.
Once I get a new laptop, I'm probably going to use my current laptop as a Windows File and print server. So, I'll share my music from my Linux box through samba then share it from my Windows file/print server, and from there to my Xbox 360.
Samba Server --> Windows server --> Xbox 360
Think it'll work?