Sheesh! I was so taken by the idea, that it did not occur to me to search before posting. :P Well, a lesson learnt!
If we can remove something easily when objected to, probably we can save ourselves a ticket / further hassles - that was the inspiration behind thinking this up.
Inflating from the passing air will lead to drag, so no use doing it. Bleeding exhaust would also lead to a rise in back-pressure, so maybe like a balloon it should be inflated and held in place, disconnecting further exhaust flow through it.
A better idea could be having a tent-like structure with the number plate and light assembly on it (a la AndrewJ's boattail), with a clear pyramid/tent shaped cover for the number plate itself. the number plate (bearing plank) would be pushed out through an electrically operated jack like structure - it would require no inflating, and will remain tout against the buffeting caused by the turbulence...
However I should probably not talk out of my hat and let it get a bit more detailed before barking out. And also should read the other thread before posting further
