Your comment made me think of the labeling system I devised this weekend (which will get put up as a PR later this week).
An unstructured, 3rd party label of another person's profile is basically a review / recommendation of the other Nostr user. It's broader than just recommendations for relays, but every relay lists the npub of the Nostr account that's responsible for it. The relay reviews could be put there. (Though they'd be mixed in with other reviews for that person/organization).