FWIW, LWN.net has a form of this, where a subscriber can share a "guest" link for a subscriber-only article (All such articles become open a week later anyhow).
It's bad form to share guest links on aggregators like HN, I don't think LWN has a quota on the number of accesses on Guest links. On the other hand, the occasional widely-shared guest link may bring a few more subscribers, but I doubt this scales.