BIAB always tries to do what musicians do...........the people who hear me play often query wqhether I'm a musician. so maybe that's why i thought the slash chord route might work. it doesn't but the obvious solution is to use a midi bass or playable RT and edit the bass line to what you want. so do the musicians think that will work?

so i got the theory wrong but i hope the solution to the OP's problem is correct.