Cover Description

I need to work with the cover art description field, for about 10,000 files, give or take 1 or 2.

Obviously I can't do all those manually. (and I don't have to do them all at once)

I don't see an %_extended% field to be able to read from / write to.
Am I missing it?

I can manipulate the field through other applications, 1x1, but I wish to do it in MP3Tag as I have been using this fantastic software for years and am very comfortable with it.

I know the frame is APIC (ID3v2.3.0), but how to deal with it in MP3Tag?

What I want to do is work up a regex or format value and code the description based on whatever existing data is already available via existing tags.