Thanks Agent_24. The verification reliability does indeed seem to be a problem, and the very distinct lack of screening on my cable probably has something to do with it
but I got wbp's firmware installed and working ok nevertheless. My clock is now auto-dimming perfectly and when I get the chance I'm going to see if I can get the GPS going too.
I think if I decide to do anything more than just a single firmware update, I will get a USB programmer.