It would be a very useful feature to have as long as it's configurable. I think PoW could be used to deter spam, not on every message but say a relay has never received an event from you before (new user), it could require your first 3 messages be PoW. Then it allows you to dial it back if you didn't receive any 1984s after your first few. Something like this, could help new users establish themselves without requiring a lightning payment.