A paywall is a wall, it means many people in many different gardens. Separate from each other. I'd have to pay each one to post for them to see my speech. Proof of work is done once, signed into the event and can be sent to all relays. The relay can reject low difficulty work. How much work you'd do is up to you. The more work you do the more likely it is to be accepted by more stingy smaller relays. That is one limitation of the current NIPS. There's no mechanism to query the relay for the difficulty they accept.