Pipeline, thank you so much for 1) understanding this and 2) sharing that understanding with the rest of us!

The new multiriff feature seems too complicated for me to use effectively. I will try your plugin multiriff technique and if that does not work for me I'll just keep doing what someone here suggested a long time ago...

1) Save final song with all tracks frozen.
2) Unfreeze and regenerate any tracks I need. Freeze and save again as a new song.
3) Repeat #2 until I get enough options and then comp it all in the DAW

I was so hopeful for bar by bar, in-place RealTrack regeneration. Oh well.