Why not just create an open bank account that users of Open Source software can easily donate to. Setup a board or advisors to oversee the account and then distribute money based on time on the list. If a project is successful it will get more funding because it will remain in use longer, or at least that is the theory.
These are totally for sake of an example
1 - 2 Months $50
3 - 4 Months $100
5 - 6 Months $200
7 - 8 Months $400
9 - 10 Months $800
11 - 12 Months $1600
1 - 2 years $3200
3 or more years $5000
These amounts could be paid month/annually/quarterly what ever seems to be the most manageable. A percentage method may need to be adopted since that would allow for the amount in the fund to dictate how much each project could even get out of the pie.
This system would promote long-term commitment to the community.
Still don't know how to determine which projects should be on the list, but it might be able to be done through votes and mailing list traffic.
There is no 100% fair way to create a payment system for Open Source. This system tries to make it easy for people that want to fund Open Source, but don't know where or how to do it. Which would include me.
It could even be suggested that Linux distributors pay .5% into the OpenBank so that they directly assist the developers who make their product possible.