I don't use XBMC, but it can act as a UPnP server:
XBMC has (since the 24th of January 2007) a built-in UPnP A/V media server (UPnP-server). Originally added to help share files to other XBMC instances as a UPnP media source, XBMC's UPnP server also enables you to share media with other UPnP-clients on your local network. XBMC's UPnP server can share files that are normally only locally accessible on the Host XBMC device, as well as metadata (summaries, cover art, etc) to supporting clients. As of XBMC v12, the UPnP server can also share files that are not scanned into the library.
( http://wiki.xbmc.org/index.php?title=UPnP/Server )
Additionally, one can set network media sources such as rss feeds, and it supports other protocols based upon backend plugins.
One potential issue you might have is that the 360 only supports a small subset of encoding formats. In the past I worked around this this using server software such as TVersity which transcoded media files on the fly. If you have the proper backend plugins, you can achieve the same thing.