Hey Genisisfactor,
Good call, and I am totally impressed on your angle of incidence / angle of relfection statement. I am an optics professor in a college, this is exactly my bag, baby! Light, sight, and manipulating the visible spectrum.
It's also EXACTLY why I believe I can do this without anything like a film or tint on the windshield as a screen (at least for starters), like I was saying above. However, I will probably experiment.
Basically, the angle of incidence needs to be greater than the critical angle of 90 degrees, which will give 100% internal reflection. The index of glass is 1.52 - 1.53. In a car windshield, there is also the laminate sandwiched in between, which would have a slightly different "N" (N = Index of Refraction).
With that being said, and rather than whipping out all my formulas, I have just angled the LCD back and forth until I found the best combination of driver visability and lack of external noticability. Just by chance, I think I found it, but it really has to do with the rake of the winshield relative to the position of the monitor under it. Each situation will be different. And I don't think it will be totally invisible to the outside world, but as little as possible.
There is more on Critical Angles here:
http://www.glenbrook.k12.il.us/GBSSC...rn/u14l3c.html