Suppose you applied hamilton's logic to a situation in which one individual is past reproductive age. could there still be selection for an altruistic act?

Yes, there can still be a selection for an altruistic act by means of post-productive individual that benefits a young relative.

The older individual or the older person cannot be beneficiary because he or she cannot have offspring. But, there is still an exception, the older individual may perform the altruistic act that costs lower than expected for that individual has already reproduced.