I think the ideal HUD color would be red. So to that end, maybe just a 3 digit large LED numbers and a small mirror would be a good experiment. Ideally it would be a first surface concave mirror to reduce ghosting, but it would have to be enclosed if used for long.
Perhaps using an atmega168 to drive the LED numbers and read the serial output from a mpguino, a-la remote display. Haven't seen one of those materialize yet (unless you count daisy chaining mpguinos).
I did pick up a couple some LEDs from the local sciplus.com for $0.75 a piece (they quickly ran out of stock), and picked up the largest stick-on convex mirror I saw at the store (couple bucks) and carefully removed the plastic. The trick now is to strip the tan stuff off the back of the mirror (paint stripper?) and I should be left with a first surface concave mirror. And of course some electronics/wiring/programming to sort out.
It isn't a priority for me at the moment, but if one or more of you think it is a good plan and feel reasonably confident you can pull it off, then by all means help yourself. Let everyone know how it goes and if you get stuck.