Why I think this suggestion won’t work is because virtually everybody has a lot of expired postcards on their way to them. Some were never sent. Some got shredded by the postmonster. Some were missent to another country and will arrive in two months. Some come from postally unreliable places and take forever to get to you. Some were accidentally posted in your neighbor’s mailbox.
A local friend and postcrosser recently moved; she still had access to her old mailbox, but decided it would be cleaner to autoregister everything travelling to her, even if she set her account to inactive plenty of time in advance. The system showed 23 cards! Only two of those actually arrived at a later date. A few had scanned images and were from well seasoned members - the rest were from people who were inactive for months, had very few or no cards sent and registered, and might have never bothered to send them in the first place.
I think it’s way more practical to send your own message when you see somebody is still active and registering postcards - but not yours. Maybe you have you received a postcard that looks like this and that, sent from this country around this date? If it rings a bell, please check, you haven’t registered it yet. It’s personal, it’s specific, it might trigger a memory rather than send somebody auditing their whole collection. And it specifically works for people who are around and still game. Somebody who lost interest or has troubles in their life won’t come back to look for a card or 20 even just because a robot email tells them to (they are already receiving it, by the way, and obviously ignoring it). I don’t blame them, it is what it is.