This is really interesting and it seems that OnSong’s approach to automation is similar to that in SLM - I’ll have to look into it. I don’t have the app with me but looking at the manual online it says that OnSong can play midi backing tracks that you can feed to an audio module / app for playing midi instruments - do you do this? What is interesting about this is whether the midi backing track follows the midi clock sent by OnSong. I imagine that it does. If so it should mean that it is possible to make the backing track play in sync with the BB when you send the midi clock from OnSong to the BB as well.
One idea I have that I haven’t tested is this: I believe it is possible to send midi notes to the BB and have the BB play them. If you use a drum set with bass, then those notes could be the bass part only. Now, I wonder if the BB could play notes sent to it from an external source, such as OnSong, whilst at the same time play notes programmed within a BB song in the normal way - whether the two would work simultaneously. If so this would be a brilliant setup: you could send the BB a bass quitar midi track from OnSong that would follow the song structure, verse, chorus etc. and at the same time play the BB song, drum parts only, that would not follow the song structure but would allow you to transition to different drum parts and add fills when you wanted as in the original BB setup. This makes the BB much more dynamic and interesting in a performance and possibly makes setting up the drum parts easier (as you don’t have to worry too much about song structure).