Firstly, Rockbox doesn't actually use the first ARTIST tag found, but the last one (which in and of itself is a problem because the last artist listed is usually just a featured artist or something). However, all instances of the ARTIST tag are read; it just so happens that the foremost ones are discarded. In fact, as far as I can tell, all tags within a Vorbis header are read--whether standard or not--before they are appropriately categorized (or not).
Ideally, it would be nice to have Rockbox handle any and all types of tags. But I agree with you that there is no clean way of doing this at the moment (particularly given the program's ID3 underpinnings). However, this does not mean that information has to be discarded. In the lowly case of the ARTIST tag, for example, I don't see why multiple instances can't be concatenated into a list of artists on the fly. That way, the original tagging structure would be preserved and information about the track would not be left out.
Personally, I'd be more than happy if I could just dump the comment header of a track right onto the WPS. I could care less about an elegant solution. I just want to be able to see all the metadata I have stored with every track.
I'll stop rambling now...