I've asked the same question in the past, and see it keep coming up. The lack of something centralised or coordinated for donations is a massive missed opportunity for FOSS.
This is exactly the sort of thing I think the FSF should take on. They are a nonprofit which exists to promote free software, and I think this should include the freedom to donate and receive funds. They are smart people and I'm sure they could figure out how to do it in an efficient way.