Most probably this is using Hands Free Profile (see sections 4.5 and 4.7), where the Audio Gateway (your phone) can report both signal strength and battery level to the Hands Free (your car).
I'm not familiar with the internals of Android, but I would imagine the Audio Gateway is part of the platform and not programmatically modifiable - here's no good reason to let a developer modify the battery or signal strength being reported to other devices.