We already talked about something similar months ago, if I'm not wrong with nostr:npub1jlrs53pkdfjnts29kveljul2sm0actt6n8dxrrzqcersttvcuv3qdjynqn. The idea was to invite people to join Nostr and do the PoW for them using a specific kind for delegated PoW as soon as they create the account, to allow them immediate access to a PoW-restricted relay.

The difficulty adjustment and forcing a redo is an interesting option.