Thanks for your elaborate response Neil.

Yes, I was able to enter preboot when the device was 100% operational (ie before I attempted the firmware upgrade based on Catymag's tutorial). It is then that the GPS got bricked.
The thing that complicated matters is that if I turn on the GPS, nothing happens. It gets stuck on the logo. If the battery has a charge, plugging on the USB also does nothing, no matter where I press on the screen.
If the battery is empty, then plugging the USB (and holding the top left or anywhere for the matter) puts the garmin directly into mass storage mode and I get it on my computer. It boots directly into mass storage. As you said, I think I got one of the stubborn units.
I'll try some more and see if my luck strikes. Thanks once more.