1) Prime lenses do not have zoom rings, because, well, they don't
zoom. I don't understand what you mean by saying the continuous
variability must be real-time.
Of course prime lenses don't have zoom rings. Why bring this up? You never even mentioned primes in your original post, so I'm not sure how you expect anyone to know to constrain your argument to them? Also, when you say "If you attach a DX lens, the crop range will be restricted," how does that imply prime?
2) The crop could be indicated the same way as in the D2Xs
viewfinder, except the size of the mask would be variable. If they
are particularly clever, there will be an electronic viewfinder
option, so the viewfinder image would zoom.
An EVF might do the trick, but for a manual VF using a D2X LCD screen, exactly how many masks would you need to have for the VF to qualify as continuous? Would Nikon still be able to claim 100% frame coverage for every possible step? Nikon and Canon are incredibly conservative with their flagship VFs, with emphasis on maintaining accuracy, size, and brightness. I doubt we'll be seeing an EVF replacing an optical pentaprism VF in this generation, and I doubt they'll fudge with it and throw some kind of zooming "lens element" into it either.
Regarding interface, if you can't control a "continuously variable crop" in real-time (i.e., setting the range by a lens ring, W/T buttons, or command dial, while looking through the viewfinder), you're pretty left with going into the menu and scrolling through a list of "crops" to choose. Since you mention it will be continuous like a p&s cam, I assume that means in a menu you'll have to choose from at least 9 steppings (1.1x, 1.2x, etc.), or probably more. I have a hard time seeing the utility of having to go to the menu to adjust between a 1.4x and 1.45x, even on a prime, so I assume any utility this function would have would necessitate it being accessible "real-time."
The problem with real-time is it would then have this huge overlapping functionality with your zoom lenses, from which you're supposed to be able to quickly and intuitively span the range, but now you rack in and suddenly you're NOT wide enough because you're at 2.x crop, and have to then go to some other button to manipulate that as well.
Finally, assuming you're only talking about primes, your reply implies that this feature wouldn't be available with zoom lenses. Why would that be? And does it really make sense for Nikon to implement such specialized silicon, create a VF with dozens of LCD masks, and limit it to use with old lenses from which they derive little/no current profit?
Everything about this points to impractical, pointless, and confusing. That's all.