Wipper snapper, I wish my experience matched yours. I'm just trying it again, and when the "Device Restart" message came up I (1) clicked the OK button in the popup, (2) disconnected the USB cable from the 990, (3) clicked the Master Power button off and then on again, (4) reconnected the USB cable to the 990. The "Device Restart" message hasn't reappeared, but the upgrade loader window now shows "Erasing" and the progress bar has disappeared, and that's the way it has been for the last fifteen minutes. Just like so many times before.
Have I done it the way you suggested?
About the only other thing I can think of now is to check the local RS to see if the RS232 adapters I need have come in yet (I have a 9-pin female-to-female cable).
This is either depressing or infuriating, depending on one's temperament.