Actally I came across interesting idea, not sure can it be done, but:
Couldnt I just read "%discnumber%-%track%" combo as %track% and then split it aftrewards by somekind of script which uses tracknumber field and - as separator? 
Or read it as somekind of custom variable and then split it to %discnumber & %track%.
Theoretically it could be possible, as it includes clear separator, which differs from other separators.
"-" VS. " - "
Then it would be possible just create Action group, which includes all the necessary steps.
Anyway...looks like a time to do some experiments on some small group of files... 