Are these still being done? I haven't been here in ages, we used to have a community run system where we would take turns in creating a backing track each month so that everyone could put their own solos over it for competition/experience/feedback/constructive criticism/learning etc. They used to be open to all levels from total beginner to seasoned pro. It was great fun and very cool to hear all of the different interpretations and directions that everyone came up with over the same backing track.
If enough people are interested in continuing this I can offer a page to upload all of the tracks to on mp3unsigned.com
PS I hope I'm breaking any rules by posting this!