Here is the Network Implementation Proposal for POW (https://nips.nostr.com/13)

Identity on nostr is just a public/private keypair.

You can enhance that identity by sending a kind0 (metadata) note with your name, bio, pictures. So i guess you're suggestion to add PoW for kind0 events which is covered by NIP13 too.

Right, so the general NIP13 could be used to define a "ID proof-of-work" note attached to an ID. You could add PoW notes after ID creation, or create new, more difficult notes if the original one turned out to be insufficient.

But although there's the technical components available to easily build such a thing, you're not aware of such a standard having been discussed, much less defined?

EDIT: To be clear, the idea for this would be a equivalent of the "blue check mark", but 1) you pay it yourself and 2) you can make it as cheap or expensive as you want.