You can reduce the transactions with payment providers. Instead of money exchanging from contributor to maintainer, have a token exchange. Contributors fund tokens with real money, and pull requests cost and refund tokens. Like an escrow account. But the money never goes to the target system. There are no perverse incentives to steal tokens. If you get a reputation of not refunding tokens (which have no value to a maintainer), then contributors will dry up.

"TrustTokens" or "EscrowTokens"