I would need my app to access the system permission write_media_storage which is accessible on by signature|system. Is there a way how to get the application signed by the firmware signature? Even if it meant having to go through some google certification process.
These certificates are different between vendors - so you would have to talk to the vendors and not google. It is not impossible to get them for small vendors ( I have it from one very small / special interest one ) - but I doubt you get it for big ones like Samsung. They can even use different certificates between devices.