Another, more "standard" approach to all this, is to record a block chord progression (in any desired inversion and rhythm) into a MIDI file. Then connect an outboard MIDI file player to drive the NTA, or even the "lower" voice. Bass inversion "off" or "on" for variety in real time. If the file player has "mark and jump" functionality, this would seem to be a pretty flexible setup. It's not exactly spontaneous. But ANY use of a C.S. means you're no longer 100% real-time. If you know that a song is part of your set, having a stored MIDI of the chord progression is no big deal. Great for practicing, and since many songs use similar chord progressions, it could be fun for jamming with a variety of styles and tempos.

To address your question about Yamaha's XF implementation... I don't think the list posted above by Tapas is complete. I'm pretty sure if you create a step recording using the CHD tab in Song Creator, you can specify slash chords. If you export from Song Creator before you "expand" it into a full MIDI, the sys-ex for the XF chords will be at the end of the file. Maybe someone here can take a look and see which byte specifies the bass note! Good discussion!!