The ORIGINALDATE (original release date) field is saved as a TDOR tag in mp3 files (it's listed under __translate in id3.py), but is not added to mp4/m4a files (it's not mentioned in mp4.py).

More generally, Picard saves all fields to flac tags, but only the known ones to mp3 and mp4 files; why can't it save all of these unknown fields into tags with the same name (e.g. as TXXX tags in mp3 files and with an appropriate prefix in mp4 ones)? Even if iTunes or other common players don't support some fields, they may be useful for other programs.

If this is not done by default to avoid creating incompatible fields, it could still be provided as an option for advanced users who don't want to lose tagging information. (or could it be done as a plugin?)

Thanks!