Juliet,
As far as the differing scans are concerned, in the setup on my aircraft's HUD system, the HUD only provides basic information and not a full PFD representation. So in the case of looking away from the HUD for whatever reason the main displays are still PFD, MFD and ED where as if the HUD was providing PFD information the normal PFD screen may have a Map display the MFD a weather radar or other data displayed changing the normal scan.
Which in itself isn't an issue unless the particular aircraft you were operating
has a U/S HUD system or you were to only use the HUD as a foul weather approach aid in PFD mode with a different HDD setup. Which on re-reading the earlier comments isn't the direction this thread was heading in.
I'd be interested in knowing the design process with the HUD interface. Is it driven purely by engineers or are flight deck actively involved in the process? An engineers view of the way the system will be used and the pilots can be very different as another thread is alluding to.
Interesting to note it is a field being driven by the Biz Jet side of the market where getting into smaller less well equiped fields (ie no CatIII etc) appears to be the driving force and I'd imagine a great selling point. "Yes our aircraft is $1mil more, but our aircraft will get you to your chosen destination and not a diversion airfield, more of the time than our rivals"