I think usually the nature of a retro fit is to fit something that car was designed for, but after it is already fully built.
What you're talking about is adding a system that was not designed for this car. It may well be possible but the work required likely makes it not worth doing.
Anyhow, who needs a HUD on a Z4, you should be having fun!
Black Sapphire BMW Z4 sDrive35i
Mods: HiFi Tweeters Stubby LED Halos & Front Indicators DVDInMotion SmartTop 313 BMW Performance Alloys LCI Repeaters http://goo.gl/PF4nSz