Since the PO cancellation doesn't always match the city that the lighthouse is actually in or near, how about something more achievable?

Buy your cards and have them STAMPED at the lighthouse using the USLHS rubber stamp? I've gotten a few of these from others in past PCEs.

If you are visiting a lighthouse that is open and buy your cards at their gift shop, I don't think they'd object to your stamping each card with the rubber stamp. (And not charge you the $1 they are supposed to collect for stamping your passport.)