It's not unheard of for contracts to be considered void if certain clauses are found to be unfair - it could be argued in court that you have paid for access to all league home games & that as such it is not considered fair that a game is excluded from such because of a decision not to take up a ticket for a non-league match, contract clauses have been chucked out for less. If you can find someone determined enough to take on United you'll probably find a lawyer willing to take a shot at this, some of them love knocking these clauses out & many more of them would love a shot at making an institution like Man Utd look bad!
And something tells me it's easier to jump the turnstyles at Crapielow than it is to do the same at Old Trafford - I mean fuck I can barely get in the turnstyles at Ibrox let alone think about jumping them (which would be impossible anyway!).