I have been working with BIAB for a while but just recently decided to use the backing tracks in a duo I am working on. First I wouldn't discount the Yahoo group for BIAB because they have a lot of songs that the chord structure is accurate. You say you want to do it your way but with doing a cover the chord structure pretty much has to follow the original tune. Once you download a song from the group then you can change key, tempo, and styles. I can get a really decent cover in about 15 to 30 minutes and it is free. I also download some free midi tunes that once you get decent soft synths and drums added sound great.
I will say this I also play in a band and on the last gig we did, me and my fiance (this is the duo I am setting up) did a couple of tunes with backing tracks. What I learned is you have to be really careful about how much you add. It gets into the karaoke world real quick if you over do it. What works best for us was basic accompaniment drums, bass, and rhythm guitar and we play piano and guitar/guitar synth. This really simplifies what you need as a backing track. I have also learned that if I can't sit down with an acoustic guitar by myself and do a reasonable version of a song then backing tracks aren't gonna fix it. Just my 2 cents.