Got some USB (type-A) connectors from ebay (10 for $2.35 with free shipping) to make my own "cal key".
First, I soldered 100 ohm resistor across pins 1 and 4, but unit didn't enter calibration mode on boot (simply booted up normally). Next, I tried shorting pins 1 and 4 and that worked!
Firmware upgrade was successful with the home made "cal key"...