Don I'm sure you know more about this stuff than me. But after looking at the manual, I think the chord mode you need is called Fingered 2. That, plus a Lower voice with lower hold turned off should allow you to play single notes in the split area without changing the chord. (I believe there's also a pedal function to lock in the chord, but you might want to use your foot switches for other purposes.)

Did you find that you have to change your learned fingering to avoid triggering a 6th when you wanted a minor 7th? I wonder if Ketron changed their algorithms to favor the Minor 7th (which seems to be much more common in the music I like to play.)