There's not much point in trying to turn donations into a forced payment system. Anyone (privately) developing open software has to accept the fact that it is generally not done for money and people might or might not donate (most people don't). If you want money for your efforts, go for app stores and sell closed software or get a job in some software company.

The problem with a "fund" is that it's a nice idea but won't work in practice unless there is a large company contributing a serious amount of money into it. If it relies on private donators the amount of money available for developers will be insignificantly small when distributed between many projects.