To get what you want, you might have to step outside of the box...so to speak... take the tracks to a full featured, easy to use DAW, and do some audio editing to get the result you want.
So... in your example... you want the bass to hold a note while other instruments play and change to other chords.
So.... in a DAW.... you simply let the other tracks play.... You insert a volume automation envelope and pull the bass OUT for the 2 measures. You insert a bass synth that sounds like the bass RT and play one note. waaa laaa.
I did that on piano in Fools Errand. The entire piano track is a RT that plays all the way through. But, there comes a part where the piano is playing single and double note chords... that's a midi piano played by me.
FOOLS ERRAND the part occurs between 3:24 and 3:53. Even an intent listener will not catch that it's two totally different pianos because I took the time to match the tonal quality of the mid to the RT.
I do not hesitate to use MIDI in this manner when I need something that BB/RT's won't deliver easily. I find I have to think outside of the box quite often as I work with music. If I can accomplish it in BB/RB, well that's good and I do that... but if I run into the proverbial brick wall in BB/RB, I figure out how I can accomplish it in a different manner and set about finding that solution. It's amazing what you learn by doing that.