Hmm, a quick thought-out solution that just popped up in my mind is of course having a framebuffer after all (or at least partially in a vertical divider sence) that lives *outside* the C65GS space (as far as the C65GS user / programmer is aware) that captures 1080 scan lines, ditches the 120 left towards 1200 and traces those at a different frequency and totally separate from the system.
As far as the user is concerned the beam tracing will have a scan frequency like it is now on par with the 1200p. Looking at the screenshots the horizontal border is very thin which suggests cropping. Losing 120 scanlines due to cropping, 60 on top and 60 at the bottom won't do any harm in that respect, right?
Of course in doing something like this we'll have to sacrifice a part of the 128MB DDR2 RAM. That would amount to 1920 x 1080 x 12 bits/pixel or a mere ~3MB for a *full framebuffer* while maybe only a couple of vertical segments are enough to sync stuff in multiple stages towards filling the whole screen.
I realize I make this sound way too easy but the beauty of above treatment would be at least that there wouldn't be any messing with the current system internally as this solution would be slapped on and could therefore even be upgraded to *post release*.
P.S. Not only could this solution be slapped on, it could also be made optional (configurable depending on monitor type) more easily!!