When one opens a camera application (like millipixels), the screen should show what the camera points at and capturing color photo and video of that content should be possible. It should run in a stable manner (meaning no anticipated crashes) and the required changes should be accepted into the postmarketOS edge branch.
As for devices, please refer to the linked wiki-page. It names one Shift device and one OnePlus device (and a variant of it). If only one device were to be chosen, it should be Shift.
I understand your concern about cost - maybe the company could be bothered to sponsor a device for this development.
There are actually mentions of people having had partial success, but your point makes sense regardless. I updated the description to clarify that interested developers have to express their intent beforehand. This way it should be easier to track who actually made it happen.