Can't all the Ketron arrangers trigger the auto-stuff from the right hand? If yes, then all you need to do is set the arranger to "right hand" and turn OFF the auto bass part. Set the left hand part of the kb split to a bass voice, and WHAM ! You have the "bassist" feature.

Save that setting to a registration and you can just switch rhythm patterns on the fly. The PA80 does this as well. Also Roland, I think. Fran?