Thanks again JJ for all the advice. I keep learning new things from you.
In case anyone finds this thread in searching for a solution to a similar issue, I will tell you the solution that worked for me.
To batch delete the ReplayGain data in TXXX:
Select the desired tracks
Right click and choose Extended Tags
Click on the Replay Gain field
Click on the Delete button (the Red X)
Click OK
Voila! All the TXXX frames are emptied (for the selected tracks)
To batch delete the ReplayGain data in RGAD:
Select the desired tracks
Hit [Ctrl]X to cut all the tags to the clipboard
Click OK to accept the cutting
Hit [Ctrl]V to paste the tags back into the files (from the clipboard)
Click OK to accept the pasting
Taadaa! The RGAD frames have been cleared.
It turns out that RGAD is obsolete, so when Mp3tag writes the tags back to the files, it simply drops them. This was the only solution I could find for batch removing these nasties, but it works just fine.