You can have multiple sgu versions of the same song allowing you to exceed the 7 track limitation and also make various changes for specific instruments to either be embellished or simplified by making chord chart changes and exporting the track to a wav file. With the Windows version of BIAB, the various exported wav tracks can be assimilated in RealBand.
For instance, a bass part can be enhanced using slash chords or adding chords to each beat of a measure to accentuate between verses or moving from a verse to a chorus or bridge while other instruments follow the original chord chart.
I've been adding tracks by using 2 SGU files the original SONGTITLE.sgu then using saving_as to create - SONGTITLE_ADDTR.sgu switching instruments in the "Band" in this file. Then drag and drop what ever new part I created. Open up the original SONGTITLE.sgu in RB and drag and drop tracks the additional tracks into the RB session.
I'm always freezing all tracks after generating the new song arrangement when I have what I want.