If it's their last feed, what are they doing for the other ones? If they're taking bottles, could you try giving them a bottle at this time and then gradually decreasing the amount of formula in their bottles? Also, since it seems to be a habitual waking problem, maybe you could try wake to sleep? I know there's some info about it somewhere on the sleep boards.
Edited to add: Just wanted to say that from your post, it sounded like you wanted to give up BFing entirely (as in this early nursing was the last nursing they had) - obviously if you're still BFing for the other feeds, you shouldn't give them formula. You could try decreasing the amount of time you spend BFing as well instead of number of ounces in a bottle.