I also tried to extract the midi from the supertrack demos .sgu folders (took a test copy of: SuperTracks8 Demos - More Country Piano, and kept a few files in it for test). I then doubled clicked in one of the SGU files to open BIAB.
Using the Midi Batch option as above it seems to generate the midi files in the same folder as I selected in the batch command and from which started the SGU from. However I got midi for all the instrument tracks not only the piano (even if I have piano selected and mute the rest).
Also comparing a midi file generated with the Batch command vs generated midi with the single file command seems to generate slightly different midi files for the same SGU song. But when generating a single file midi I can get only the piano track midi when I have piano selected.
I now wonder if it is possible to only have the "piano track" generated as midi when using the Batch command.