Hi Cyclone,
Yeah I think we are drifting slightly but hey, thats the joy of Prune!
I think what you are saying is that where the HUD is the PFD, and the head down displays normally do not show any PFD info, the loss of the HUD would mean an unusual head down set up?
This situation on my old type leads to a PFD in front of the pilot instead of the normal nav/radar screen. Normally in this case the moving map display would be swapped for a nav/radar display for that pilot, so that they stay fully in the loop. The 4 screens would now be configured as (left to right) a PFD, a nav/radar, engine info, nav/radar (assuming a failure of the Captains HUD). Bear in mind of course that in this situation the pilot that loses his HUD would hand control to the other pilot so that they continue flying using the HUD. Preference is always for a pilot to be on the HUD rather than a HDD PFD.
Your last point is, I think, bang on the money!
Quite interested by this as my HUD experience seems to be in a totally different fashion to most others.