Super simple inverter-based relay bypass

[R.G.]

I'd do a PIC or Arduino/ESP that then whacked the floating switch. Not a biggie, but the interface is considerably more complex than the whole relay interface. I'm pretty sure there are existing libraries. Here's a breadcrumb http://www.rossbencina.com/code/midipic

In fact, the MIDI interface inverts the intent of the design: it becomes a MIDI receiver that happens to also accept a local switch closure to flip the relay state. The inverters and so on become parenthetical.