I think this is the perfect application of a micro payment service. Each PR must be signed with a nominal amount of money, say $0.15 give or take. You send in a commit, with no expectation to get it back.
I think this idea doesn't go far enough. If it's money that's motivating slop, fine - let's make it about money. $50 to submit a bug report. If it's legitimate, we send you back $60; the judgement is on the curl maintainers' honor. If it wastes time, well, at least the curl maintainer gets a steak dinner.
I think a deposit would be better then. You make a deposit of ¢15 for each issue and PR you open, and then the maintainers can decide to give it back to you if it was opened in good-faith/useful/merged/whatever policy they decide on.