If the key is managed by the OS but is inaccessible to the user, the concept would seem to be incompatible with free software operating systems. Also, if the camera app can have anything it "makes" approved, the app itself could take a deepfake video (from the web or device storage) and have the OS sign it.
The only way I see it working is if the key is "burnt-in" to in the camera hardware and any applications cannot MitM it.