Wow, thanks everyone - I tried all the Simple Arrangement settings but it didn't consistently affect the offending RealTrack because (I reckon) even if its 'Simpler Available' column (in the RealTrack Picker) had an 's' in it (which it didn't), as pax-eterna mentioned, Simple Arrangement would probably just affect the "busy-ness" of the playing, not the chord.

I did seem to have some success with the "Jazz Down the chords" option that Graham pointed out, in that the 7ths (e.g. G7) were now gone. Problem is, it now generated unwanted Maj7 chords elsewhere in the track(s).

Also, a lot of this - even were it to succeed - would require regenerating the whole track anyway, most of which I like, just as it is. You know what that means - I'll have to resume using RealBand, where I can regenerate on a bar by bar level. Shame really, since I like BIAB's look and interface much better...

But I appreciate all the great suggestions and the detailed illustration of where to find some of these options. This will be very useful in other situations in the future.