very nice program. one thing you could add is an option to change the encoder flags, like copyrighted, original etc.
this feature is not planned.
Implementing this feature would not be that hard (but not easy, either), but it will take a lot of time for the files to be "tagged", as MP3Tag would have to navigate through all MPEG Audio frame headers, read their information (bit rate, sampling rate...), modify them and finally write them back.
actually there is one program that does this (mp3 tag studio, i believe). it's pretty fast, too. i understand if you do not want this feature.