Hello Paul,
Do you mean something like Mood, Style can be considered by some as a sub category to Genre. Whereas Genre would be the top level and Style, Mood, Etc., would be lower Genre?
If I've got you right, then I can't see why not. I you can create your own tag fields too and call it what you like.
I've come across "Discnumber" and "Compilation", but I've only seen them as numbers, then there is "Albumsort". Where you could use the album as "Now That's....", and albumsort the actual disc title "Now 199" or what ever you wanna use.