The intended use for the mask is with height fields, I would like to use satellite photos as a base to build up masks for scattering and texturing.
I would need an inverted chroma key operation but with a black and white result I imagine.

Maybe I'm just blind and this is in some tutorial out there, if so please show me the way

You would do the keying operations in a COP2 network. Set the cop to output a volume slice on ZX plane. Generate a mask plane in the cop with your desired mask content. Then in sop combine to terrain using a heightfield layer node.

This should help you out: https://hdbp.io/lQ1wtHD2 [hdbp.io]