[Fiddlebog]

I've considered doing a paddle mod for my tip Cayenne. I would manufacture my own paddles and such. I'll follow up if that ever works its way up in my queue of projects.
As for the original question, the answer (IMO) is simple:
relays that cuts the ground to the switches when in manual. May be able to tap in to existing hardware, or just slap a limit switch in there.