I can't believe I didn't find the answer to this somewhere on the net. But here's what fixed this for me:
The Update Disc has to be created on a CD-R and NOT a DVD-R
Since the RHB disc drive is a DVD drive and I happened to have a pile of DVD-R's handy, I orignally made an update disc on a DVD-R. And as mentioned before, this worked fine when I first inserted the disc but then froze the radio on the logo screen when it rebooted.
While playing with my "bricked" radio for a while, I noticed that it did sound like it was trying to read the DVD when I took it out and reinserted it. But nothing happened. I recreated the Update Disc on a new DVD-R at the lowest speed, But still nothing happended - radio was still stuck.
So after considering what might have happened and what other options there might be, I decided to find a CD-R and recreate the Update Disc. When I inserted this new CD-R update disc into the radio (with the Jeep Logo showing on the screen and the key turned on) the drive spun up and.... Voila! The update program loaded and continued on through the process like nothing had ever gone wrong. Whew!
My RHB is now successfully updated to 23.04.41 and Travel Link is working.
Thanks for the replies!