I have asked the same question on the XBMC forum...
My setup:
I have a setup with linux/xbmc as htpc and a debian server which stores all my media and shares it across the network. For movies and tv-shows I let xbmc make a library centralized on the server, for music I already have a subsonic server with it's own library. I am very satisfied the way subsonic handles all this and I don't want to maintain a second library for music within XBMC. For this purpose I use the Subsonic add-on in XBMC frodo. However I am missing a lot of features compared to a library in XBMC....
Question:
Is it one way or another possible to replace the XBMC library with a subsonic one?
or
Am I missing something and are there other alternatives?
or
Is there anyone who would like to mod the current add-on and add more functionality?
My first question is imo the best integration, but don't know if it's possible?