Using a projector would be a bad idea in my opinion, driving down a road w/ single hard point of light coming from your dashboard that would be visible from the outside would only cry out.... pull me over!!!
As for a hud.... its been discussed many times before.... how would it work... pretty much go w/ the idea you have, get a screen put it in the dash and then tint or creat a reflective surface on your windshield somewhere.
Now here is where you run into a few problems.... number one being that you will have a screen in your dash visible to the ouside world especially when its on and when your driving... this will cause you to become a target to both thieves and police!!! -Solution to that would be to basically to put it a few inches down in the dash in other words create a hole!!! That would make it a little less likely to be seen from the outside. The other problem I could see is if you actually have problems w/ the screen it would be a b17ch to get out of there!!!
Anyways there is a ton of software out there I think if you search it on the forums you'll find info on rotation software otherwise check

!! Good luck if you accomplish the HUD post step by steps!!!