Thanks for the info. I believe I have found the reason it is not working. I just tested the sled harness with my multimeter and sure enough:

Sled Pin 8 goes to RS232 Pin 3 (TX)
Sled Pin 9 goes to RS232 Pin 2 (RX)

which is the opposite of what the wiring diagram in the FAQ says.
Perhaps somebody should update it.

This is of course assuming that pin 8 is meant to be RX and pin 9 is meant to be TX, hence the system being designed to be a null modem. Can one of the designers confirm this?