Don’t I know it! I’ve been battling this problem for two and a half days! xD
It’s also a pain when you have to work within specific limitations of how the scene is set up.
But it feels like it should be possible to make this with a stencil buffer. I already know that I can use a simple texture mask to make parts of my final rendered image transparent again, even with the use of a ppm. So it’s just a question about how to make the mask out of what’s already on screen. A short term, more brute force solution, is to make masks for the different camera angles that I want, but that’s not very neat and absolutely not scalable xD
With the videos you provided I should be able to do a proper solution so thank you very much for the help! I also realize that I need to step away from the complexity of my scene and try this out on a much smaller scene if I am ever going to wrap my head around this! 
If you want I can update you if and when I manage to solve it?