It seems the first door to unlock is finding the method to get the dx(g) into download mode. I would prefer to use ATK for recovery, it seems more elegant than using the serial port, and I'm going to at least try a few key combinations in the hope of hitting the correct one. Then it would be down to grinding through the ATK manuals to hopefully find the right settings for the i.MX31.
However, I am glad the serial port method is available. Not a big problem to get to find the location on the pcb, per
https://www.mobileread.com/forums/showthread.php?t=49942 but I am not clear whether it's OK to use 3.3V levels or the 1.8V that the K3 needs.
Thanks for the advice, I feel like I have a direction and a road to follow...