This is the most difficult part. 

A brand new npub doesn't have a wot network from his pov, but that's not a big deal - onboarding client should ask them about their content preferences/hashtags and even if user doesn't choose to follow someone and get his wot seeded, client could use some popular accounts from the topics user has chosen to serve as temporary wot sources.

The bigger problem is that relays can't distinguish a brand new npub from spam. I.e. if someone big tweets about nostr and a wave of new people comes in short time, it's indistinguishable from a spam attack from a botnet. And pow doesn't help much - you'd need something like 100 seconds of mobile cpu to produce pow equivalent to 1 sat - and I bet reply-scammers earn way more than 1 sat per event they post. 

I keep getting back to this issue in my head from time to time, and still can't find a good general solution, only whack-a-mole. Users can be protected from spam by wot, but public relays designed to onboard new users can't. Unless we attach some extra signal to new users (pow? 1 sat? some version of your pow endorsement?) while keeping the friction low.

Any ideas how "pow endorsement" could be practically applied to onboarding users at scale with low friction?