There is a workaround to getting the Sirius broadcast to be accessible to XBMC, but it involves installing two pieces of software on your computer and dedicating the soundcard. I installed Simplecast and a Shoutcast servers. Simplecast encodes whatever is playing through the soundcard into a broadcast file and Shoutcast server distributes it over my home network. Then I put a playlist file (.pls) in my SMB shared folder, configured the playlist for my computers IP, and disabled some of the relay features in the Shoutcast config file.
Open XBMC, point it to the Playlist file and it should work. Im still a little fuzzy on proper setup of the Shoutcast server, since I dont want it accessible to the outside world or to accidentally broadcast to a relay.
Downsides are:
1. It ties up your computers soundcard.
2. You cant change the channel without going to the computer, radio, or satellite box.
3. The XBMC playback doesnt sync with the soundcard (due to the encoding)
Upsides are:
1. A simple computer with soundcard (no monitor) could run the system
2. It plays either Sirius home unit, Dish Network, or internet stream
3. Can be used for any other easy plug and play sound application
It might be easier to install Linux on the Xbox and access the internet Sirius through a web browser.