The protocol is in the PIC interface documentation which comes with FMS 2.07 (and anyway it's pretty easy to work out from the PIC source code, in the same place).
But I'm not sure why you want to use the Rx. Presumably that would mean you'd need to take at least 4 outputs and combine them back into a meaningful signal on the serial port. It also means you need to have the TX transmitting which means the battery won't last nearly as long etc.
Still it's an interesting project. Good luck.
Steve