The gross inequity in parks funding is a real issue, and one which materially harms poorer and outer-borough New Yorkers, even if one were to accept the argument that Central and Prospect Parks deserve more money because they bring in tourists. I suspect that this particular approach won't pass legal muster, but I do support efforts to try focus more attention on the smaller and more outlying neighborhood parks. I'd start with a PR blitz to try and get people to shift their donations to the smaller parks, as well as moving the Parks Dept. operating budget in that direction.
(Also, this is a great example of how and why private charity is a pisspoor substitute for public spending.)
It's a worthy cause, I agree. It's just that the proposed solution is wildly unconstitutional. Your idea sounds like a solid one to try to close that gap a bit.