I think a really good solution would if the TRAX developer could implement midi to allow you to select tracks, start / stop and jump to markers etc. using midi commands from an external midi controller. Then the backing tracks could be controlled from within SLM and if SLM had the ability to implement markers in the automation sequence and jump between them, an easier option for SLM as Arlo mentioned, this would make for some really exciting possibilities.
I downloaded the TRAX app and tested it with a Neil Young song I had in iTunes. It is a little basic at the moment but setting up the markers and moving between song parts really works: I set a maker at the beginning of each verse and then was able to jump from the intro to the second verse, missing the first, without a noticeable skip in the playback. As I mentioned, MainStage has a similar capability in the playback plugin. I imagine the scenario during a live performance where you start a song with a a repeating opening riff, with the backing track playing and looping round the opening sequence, such as a simple drum sequence. As it's playing, you introduce the song and get the audience going, and then when ready, kick off the rest of the song with one stomp on a midi foot controller. The backing track kicks off with the rest of the song (and the BB, if used moves to the next song part). You could have song parts that included other instruments that you could introduce on the fly if you wanted. Definitely worth exploring.