It's a planned feature but isn't a high priority. It's hard for us to get to it with all the other things we want to implement. We'll add it one day. We need to keep hiring more developers and expanding to do more.
I have a question you may be able to answer. Are contributions to GrapheneOS in general welcome? If yes, where would one start, is there a page about setting up a development environment or outlining a list of features/areas that could need work/help? I have quite a few spare Pixel phones that are currently just idling but I'd love to try building and installing GOS myself to start out.
It uses the phone's microphone (Android OS limitation), so if it's Bluetooth in the car, it should pick up everything (just like the people in the next car over can hear your conversation, fyi), but if it's a Bluetooth headset, I don't think it will pick up their side.
Audio quality is very good. By all means test it out in different configurations before relying on it.
A native implementation would still be better. I hope the GrapheneOS devs can pull it off.
I'd love to make the switch.