Scary moment, but the code needed to do the update is separate from the main firmware and is not erased. So you do get to try again if it goes wrong (I had a moment like that when testing an early version of the custom firmware, because I hadn't updated the checksums properly in the firmware file and it was rejected after going through the whole process!).
So did it work?
Yes it did. Basically the sleep and power on combo didn't work. I had to do the power cable trick which brought up the updater. This time, I used an old DVD player with optical out. It worked like a charm this time. I have my receiver back :-) All ready to rock once more.