xorg.conf puzzle: start low Virtual resolution & switch

[Dr. Z]

Is it possible to do this with xorg-x11:

Start up in 1024x768 virtual resolution, then use xrandr or Zoom (ctl alt +/-) to get 1400x1050.

Modes "1024x768" "1400x1050"

starts in 1024x768 but with virtual resolution is 1400x1050 (i.e., xrandr reports 1400x1050).

Virtual 1024 768

makes 1400x1050 unavailable.

I'm trying to do this to work around a problem in the Xv extension, where Xv does not work in resolutions higher than its maximum image size (for me 1024x1024).

Thanks for any advice.