I know this adds complexity, but one way to handle this is simply transparency, and another way is to add choice: on the donation web page, give donors a choice if they want their money to possibly be used to fund other, related projects, that might be short on funding, and then list those projects. Funding ffmpeg, for instance, seems fair since it's a dependency. They could also just make a selection matrix on the donation page, allowing people to select allowed recipients, so for instance they could allow the Android client, and disallow the Windows client.

The Mozilla case shows why it's bad to not have transparency.

Wikipedia is much worse about it. They even brag about the other charities they've redirected our donations to.

That signaled the last time I would ever donate to Wikipedia.