a) Transflective screen will help in-car usage too.
b) You really don't want to be leaving a bluetooth device in the car. For that matter, a GPS mount is an inviting sign for thieves. Something they can see on a BT scanner tells them there's something valuable and portable inside.
c) Hey, using an external BT GPS + N810 sounds cool, but you have range issues. If you use two n810s (or n800s with BT GPS) and an ad hoc connection, you'd get more range, wouldn't you?