Grimrock

If I have a place where I plan on reusing the key, I'd most likely have that script tied to all doors except the last one, making sure nothing required is behind the others. But your most recent suggestion does make me rub my chin at the idea of a multi-cell "dungeon" area of a level where you can use the Cell Key to access more than one cell, but it breaks before you can unlock them all. Choose your doors wisely. . .

Give it a random amount and you'd really confuse people. "I opened 3 doors and my key broke on the cell with the [item] in it." "I opened 5 doors, including that one, but my key broke on this cell." "I wonder if it uses a sequence and you just got lucky?" and so on and so forth. . . Randoming though, will just make some people mad at you for letting chance separate them from the 'lucky' ones who get more from the key. . .

Charges

I tried to do that with a different idea, and the charges showed up on the item, but I couldn't figure out how to use them.

{Edit:} But that was an item that was to be used itself, not by using it on a lock. I know you could tie the lock to a script that checks for the item on the cursor and decrement the charges. My charged item didn't have an external influence.

Can you hook either the used key or the lock so it returns false AND opens the door? That way you don't really "use" it, but trying to use it has the same reaction?