I wanted to be able to preserve the cover art when clearing the tags. Here's my implementation:
https://github.com/m42i/picard/commit/d0ee1523410f1ce3165da51d75e1e280325148de

Things to be considered:

• Only works on id3 atm.
• Is having the string "coverart" as tag acceptable or should it be a checkbox?
• How do the settings "Embed cover images into tags" and "Save cover images as separate files" influence this? Atm. they don't.