Turns out the Drum Rack choke groups send a MIDI CC 120 message to all MIDI instruments in within that group. In one of my cases, KarmaFX doesn't seem to recognize CC 120 "All Sound Off", but it does recognize CC 123 "All Notes Off". I created a simple M4L MIDI module that routes CC 120 to CC 123, and now my choke group for KarmaFX works as expected.