Quote - you mean they always hafta be normal to the line-of-sight of said camera?
assuming said camera is fixed in space and only rotates?
that would mean they are always in centre-frame IMVHO.

Billboard objects are common, especially in games.  They face (are normal aligned to) the camera no matter where it is.  They don't have to be in the centre of the frame, as their orientation should change dynamically as the camera moves.  When animated, it can be very easy to spot them, especially if the camera's elevation changes.  I fully understand the practical limits of billboards, but not how others have successfully managed to create them for use within Poser.