Here in the US it is illegal to put anything in the front view. You can't even have anything hanging from the review mirror because of obstructions of view. They can even give you a ticket for a cracked windshield, but check out the new Corvette, they have this **** build in. Would be good to research this.
Here is a Quote from a Glass Manufacturer.
What makes the HUD glass different from standard glass?
All USA DOT approved windshield glass is a composite with a thin plastic layer between an inner and outer layer of glass. HUD glass has a special thin transparent laminate between the glass in the projection area. This laminate is manufactured specifically for the HUD and has different reflective properties and is more consistent in its density than the standard windshield laminate.
On some early factory HUD glass there is a rectangular area visible in the glass, at some point GM changed design and you can not see the HUD reflection area outlined in the glass with the car off.
Our aftermarket HUD glass is the same as the later GM design in which the outline of the HUD reflection area is not visable with the car off.