I just got a reply back from Andy Ritger about this, in case anyone else has this question:
I cannot think of a good way to automatically acheive what you describe
with the driver as-is today.
The best I can think to suggest at the moment is to omit the 4:3 modes
from the MetaModes list, so that they do not perturb the virtual screen
size. You could possibly setup your X configuration file with two
Layout sections, such that each Layout section points to a different
Screen section, and one Screen section lists the 16:9 modes and the
other Screen section lists the 4:3 modes. It would require a manual
step of invoking X with the '-layout' option to select the desired layout.
Another possibility would be to write a custom NV-CONTROL application
that queries what modes were validated, constructs a MetaMode
based on your search criteria, and then switches to that MetaMode.
samples/nv-control-dpy.c in the nvidia-settings source may be a good
example for that sort of approach: