And by the way of a BTW, Waydroid has pretty much the same limitation. I was hoping to use an OBD adapter to change my door chime to Rolls Royce mode, but no way without an actual Android device. 
My assumption is that most/all of these special purpose devices want to talk Serial Port Profile so they can use a custom protocol of their own devising. Makes sense in a way, I supposeā¦I seriously doubt the Bluetooth working group has any interest in putting resources into developing a (e.g.) OBD profile for such a niche operationā¦