Forcing donors to fund their wallets with ACH transfers seems like an easy way to sidestep a lot of this (except the trustee issues).
Mind you I had an advantage in that my scheme, payment and usage were tightly linked. I had a protocol that could prove a user had visited a given website, without needing to reveal that user's identity to the website and without the ability for the user to falsify visits. But with opensource software there's currently no good way to verify usage, there's no expensive signal that the code is being consumed. All that's left are cheap signals, easily gamed, like github stars.
At $DAYJOB I've pitched a concept of universal asset accounting which would actually make such a consumption signal possible in theory. For inspiration I was looking to inventory accounting and cost accounting, particularly resource consumption accounting.