If someone were to go ahead and build the proposed sponsorship platform, it ought to have some method to identify dependencies - either through developers manually specifying them, or through automated analysis of package manager dependencies etc etc - and somehow automatically allocate some percentage of donations accordingly. No, such a system wouldn't be perfect, but it would be better than nothing.
It would certainly be easy to implement "take $X (or X%) out of the money I receive and give them to foo instead". Of course, that would only work if both projects were using the same platform.
