Hi Steve,
have you possibly considered another option like Band in a Box for accompaniment ?
http://www.pgmusic.com/bbwin.htm

unless of course you really want to play a realtime arranger.

Band in a Box has styles. Midi & Audio.
Type in chord progression it does a backing track for you.
If you want to do further editing , it comes with a pc sequencer called Real Band. Load in the song you've created in BIAB, change your bass, drums, piano whatever. The styles used in
Band in a Box are also used in Real Band.

For instance, just say you've got your song recorded, but you decide you don't quite like the bass line, you can literally
regenerate the bass line with the bass from another style, you can redo the whole track or just a few bars, or you could manually edit the track.

You could use any keyboard you like with it, arranger or synth, or controller. For the audio tracks you'd need a reasonably good pc or laptop. If you don't have a soundsource you could use a softsynth that comes with it.

If the korg won't do what you want, the yammie is probably even less likely.