Basically my idea is that you buy an Arduino and connect it to MIDI in and out on your Genos with MIDI cables. If you don't have an Arduino, but have your Genos connected to a PC (or even a tablet), I believe you can do this for free.
To keep it simple, I'm just going to talk about the Right 1 part, but the process is the same for all four parts.
First you need to turn off the local sound of the Right 1 part on your Genos, so pressing a key doesn't actually make the Genos play a note.
Then you need to make a very small program that works like this:
Let's say you select an electric guitar with a distortion DSP as Right 1 on your Genos. The Arduino sets song channel 1 to the same guitar with the same DSP.
You then play a note. The Arduino sends the same note back on song channel 1, which plays on the Genos with the correct guitar and DSP.
You then switch to a piano with the damper resonance DSP. You won't hear anything different, because the local sound on the Genos is muted and the guitar note is still playing on song channel 1, which is unchanged. The Arduino sets song channel 2 to your selected piano and DSP.
You then play a note. The Arduino sends the same note back on song channel 2, which plays on the Genos with the correct piano and DSP. The guitar note you played is still playing on song channel 1, but any notes you play from now on will play on the piano. When you want to, you can select a new instrument and DSP, and the Arduino will enable them for song channel 1 and switch back to sending notes to that channel instead of song channel 2.
That's pretty much my entire idea. I have tested all parts on my PC (receive instrument, DSP and DSP settings, set instrument, DSP and DSP settings for the song channels), so I don't see any reason why this wouldn't work. I haven't made the actual program yet, but I will, unless someone else has already done it or tells me why it won't work. Any input?