Switching the boot mode to bcdw didn't change anything, but I have managed to make some progress...
I noticed that when running UBCD.BAT, it displays a list of available drive letters just before displaying the App dialog. In my case, I'm seeing:
Note the lack of letter assignment for UBCDDRV.
After dismissing the App dialog, if I run CDROM.BAT manually a CDROM driver is loaded, and T: assigned to the CD drive. Subsequent launches of UBCD.BAT and the selected application then work perfectly.
I have observed CDROM.BAT being called during the boot process, however it would appear from that invocation that no CDROM driver is being loaded. The message "CDROM: Starting (version 2.2)" is displayed, yet that's as far as it goes.
So it would appear that I have a workaround for the moment. Any suggestions as to how to determine why CDROM.BAT will only load the driver when invoked manually?