Exif data aside, metering in A or M mode should be easy enough, because in these modes, the camera only have to deal with the brightness of the entering light and adjust the shutter speed accordingly (in A mode). In a nutshell, it is a stop-down metering which is the matter of the camera. Unlike the S or P mode, the camera doesn't need to operate the aperture in the lens. Another advantage of mirrorless cameras is that they don't need to worry about the stray light entering from the optical viewfinder at all, which should make the metering easier.
I've read somewhere that Canon uses the very common RS232C protocol for the communication between the camera and the lens. The relative easiness of adopting Canon lenses on Sony bodies suggests that Sony also uses RS232C protocol which may be easy to reverse-engineer. Nikon should use more proprietary protocol.