I'm assuming you have a complete song part of which is a bass solo. is the bass a BIAB track or are you playing it live? BIAB doesn't do improvisations unless you count regenerating tracks. (Or did you mean a bass solo generated by BIAB? - if so see later)

If you have a complete song and you want to improvise a solo at a set point you will still have to fix what the drums play - BIAB and RealBand can't 'respond' like a real drummer.

My first response is that it would be easier using RealBand and midi drums. Unfold the song so you get a long file with a single chorus. Save it and open in RealBand.

Go the bass solo part of the song. delete the real drums for that part only.

then decide what you want to hear from the drums. Choose a new midi track and using the drum grid editor 'fill' each bar or bars with what you want.

If you meant a BIAB generated solo, i think you should generate one you like then freeze it and use that to decide where you want the drums to play in relation to that solo.