Plugin: "View script variables" 0.7.  Tested with both MP3 (ID3v2.4) files and FLAC (Vorbis comment) files.

When an ID3v2.4 file is dropped on the left-side, I can see my custom TXXX fields in both the bottom pane and inside the "View script variables" plugin's "Tag variables" section.  e.g., when I use TXXX:COVER ART DIMENSION in my file, it shows up as "COVER ART DIMENSION".

Once the file is moved to the right-side, my custom "COVER ART DIMENSION" field is still visible in the bottom pane, but it vanishes from the "View script variables" plugin.  I don't know if this is an issue with the "View script variables" plugin not handling a file being moved from the left to right side, or if it is an issue with Picard.