I want to say there was something similar mentioned in another thread a few months ago with a similar functional limitation .
still looking for that other thread but this seems like it might interest you also:
DIY: OT Pickup Machine footswitch (now with final pics!) - Elektron Gear / Octatrack - Elektronauts
I found the other thread but it was regarding pressing the func button via midi, the advice was ultimately send OT an update via sysex while monitoring and see if you can get the initial string, then (in your instance) start poking around to see if you can identify whether or not the the REC3 button is accessible via a sysex message.
sezare56 posted this which might be initially useful if you want to go this route.