Just FYI regarding "song (specific) styles"-- It's possible (at least on Korgs and Yamahas) to designate a melodic accompaniment track as "no transpose." So you could record a MIDI bass track unique to a particular song, and the rest a "live" style, like playing with a real bassist. Add rootless smile chords and you're good to go!

I'm not sure whether this would work on a BK with the proper CC inserted at the head of the style track, but I tried it on my E-50 without success.

Ketron has "Riff mode" but I think this just means an extended fill consisting of accompaniment through chord progressions typical to the current style genre. Their manuals aren't very descriptive and I've never even SEEN a Ketron in person except at Don's arranger jams.