It'll always displayed in upper case in the extended tag dialog. But if you like to have the field names stored as mixed case in the tags, you can use an action Case Conversion for _FIELDNAME to change them.
It's one of the available fields (besides _ALL, _FILENAME, ...) for the action Case Conversion and applies the case conversion settings to the field names of the files (which of course makes only sense with APEv2 or VorbisComments).
What I'm trying to say there is that despite the fact that only common fields are listed at this help page (e.g., %artist%, %album%, etc.) all possible fieldnames can be used in placeholders and be expanded to the field's content.
I dont think I am using any other action but the one you suggested. Is it doing something that I dont know about? does my mp3tag version interpret case conversion fieldname to also include the field content? right I have just updated to 2.46. How do I avoid changing field content when doing a case change for fieldname?
Note that I have found the "Case Conversion" didn't provide an option for me to change the atom name to mixed case the way I wanted it... For example, setting ITUNEXTC atom to iTunEXTC (Please correct me if I missed it).
Maybe in addition to the existing options present (upper, lower, mixed, sentence), a new "as typed" would be useful?
In the end, I managed to do what I was trying to do, by using the "Replace with regular expression" method, with the following options:
Regular Expression: ITUNEXTC
Replace with: iTunEXTC
(Note that for some reason, the non-Regular expression version didn't work for me???)