Japanese MSX user にがHP has made a limited-issue cart called P6/MSX, which has an OPN2 on the same I/O ports as Makoto (14H-17H).

This will need timer-based detection, and it also needs to pass a negative check for Makoto first. Not sure if the status register can be read from I/O port 16H, the datasheet says “inactive read” so if that’s true then the detection should use 14H instead, though note that it conflicts with the Music Module MIDI input.

When present, it can be used to play OPN2 music. PCM should be sent to the OPN2 rather than the turboR PCM.