Thanks to some firmware changes gone wrong, my router became unresponsive, and wouldn't even properly boot. I wasn't even able to get the failsafe mode working. I had planned on eventually adding a serial port to the router, but hadn't done so yet, and wanted to get it up and running asap.
Digging up an old Keyspan USB-Serial adapter, I stripped an old phone cable and soldered the wires directly to the pads for the (missing) RS-232 connection on the router, and soldered the other ends to appropriate pins on one of the chips in the Keyspan adapter, and (much to my surprise) started getting serial output. I'll post pics and some additional details in a future entry.