seems kinda obvious to me, but why not use the serial port? Seems like that would be the easiest thing to hack, rather than screwing the the mic or other input line.

Well, we obviously don't want to tie serial port up, but IIRC RTS/CTS pins are connected and serial driver does not use them. Perhaps they could indeed be used to recognise 'home sled' environment.

Hm, on further reflection, I think that handshaking lined are connected to DB-9 on the empeg itself, but not to the one that is part of the sled wiring harness.