How to add / convert album art to COVER (FRONT) tag?

[dano]

But then you're still limited to a fixed number of cover types. I wanted to try and name the tag 'Cover (Front)', just like WMP 11 is said to be doing it.

The ID3v2 tag standard defines only a fixed number of cover types. They are also not saved as strings but as hex numbers.
That means 'Cover (Front) is the same as 'Front Cover'

You could try to change the tag version in Mp3tag to the most compatible one:
/t/2785/1