Angle of Attack on a HUD is the difference between the Flight Path Vector (Indicator) and the waterline marker (aircraft symbol). If the HUD has an Angle Of Attack Indicator as well, it will represent the angle between your FPV and Waterline marker.
Ask anyone who fly's a HUD.
Unfortunately, I don't think I can post links yet, but please google "Angle Of Attack" + "Flight Path Vector" + "Pitch" and post the link of the second hit which is a pdf from Boeing titled "What Is Angle Of Attack".
Here is a quote.
"Most commercial jet airplanes
use the fuselage centerline or
longitudinal axis as the reference
line."
Of course we all know the definition of Wing/airfoil AOA. The questions asked above are in reference to the aircraft as a whole.
Scroll down on your aerospaceweb source with the picture of the 747.
"The following picture illustrates the definitions of angle of attack a, measured with respect to the velocity vector, and pitch angle q, measured with respect to the horizon."