Just pre-fund with kickstarter or similar.

Or through a light taxation system for biggest companies. If say .01% of the revenue of the 100 biggest IT corporations was distributed yearly among the most useful projects, those in need of maintainers etc, the developers living in bad economical conditions etc. the problem would probably cease to exist for thousands of developers while those being taxed would barely notice some missing change from their pockets. Not that different from public healthcare, therefore it requires a change of perspectives (the common good) to fully understand it.

Who defines the most useful projects? How do new projects get their start?

I think a simpler and less revolutionary route would be to simplify the registration of open source projects to be able receive tax exempt donations. That’d be a massive boost with only a minor change to the tax code.