Did you try adding custom fields with these names?
I'm afraid custom tagging shouldn't be the best way to cope with the problem... it is just a way of circumventing a shortcoming of the software.
Think about "interoperability", for instance.
Sources of commercial music would tag their files correctly (that is, according to the standard tags) if enough software (players, library managers) supported them.
In fact, I note that classical music I bought from emusic.com comes correctly tagged for Composer; however, as they know most software ignore that tag, Composer info is repeated in the Artist tag (as for the missing artist information: well, it appears in the folder that contains the music movements).Abiding to the standards, that is, allowing users to deal with ID3v2 tags as they were designed to would make library information more consistent and reliable.
As for now, among well known software that classical lovers can use:
MediaPlayer10 supports
Composer, Subtitle and Conductor (not Band)
MediaCenter11 supports
Composer (not Subtitle, Conductor or Band) and feature richer.
If MC at least matched MediaPlayer in tag support I think that (to my knowledge) it would go straight to the top of the list, besides contributing to the correct use of ID3v2 tags.