I don't think the soloist feature works in RB like it does in BiaB (not sure, never use it)
You could try a realtrack that is a solo style. Generate a realtrack and look for the solo instruments in the list. There is guitar, sax, fiddle, etc

or take the file back into BiaB to generate a midi solo.
Save it as a mid file from RB and open in BiaB. Use soloist feature, and then generate the 'solo' as a wav file and bring back into the RB seq file by using 'import audio' or bring the solo midi track in using 'load scrap'.


I do not work here, but the benefits are still awesome
Make your sound your own!