What about HashCash, where proof of work increases with more pull requests from a user ID? Beyond typical submission frequency, proof-of-work would become exponentially more difficult to prevent spam, helping to keep the riff-raff out.

Doesn't require money, just computing power.

https://en.wikipedia.org/wiki/Hashcash

If HiveCoin were still around, we could donate the hashing power to some tech-related non-profit.