I've had various situations where I need an extender on one plane and not on another. The FA on one flight said that they sometimes have to replace the buckle, and simply cut the buckle off and stick a new buckle on the now shortened end. It would suck to be denied an exit seat because of that, but those are the rules.

You might try asking the FA when you first board and are at the front of the plane that you are pretty sure you need an extender, and see if they'll just hand it to you (make it seem like you are trying to save them a trip later). I've often thought of trying to buy my own extender from somewhere but not all buckles are the same. Of course, this does circumvent the reason why they supposedly don't want "people of size" in the exit rows.