I agree! If you were visual with the field at closing time and I were a controller there, then I would have stayed those extra few minutes to allow you in
Would you have made that decision on behalf of the fire crew, the security guys and the guy in the office who takes the landing fee without consulting them? Particularly if the call was at one minute past closing time and some of them might already be on their way out of the airfield?
Personally if I'm walking out of work and the client says "can you stay for a while to do x" then the answer is usually yes provided I've got nowhere else to go in a hurry ... so sometimes yes and sometimes no, and I do of course bill for the time worked.