You could fake it with black bars.
I had an example a while back that showed how you can change the x and y zoom by different amounts to go from 16:9 windowed to letterboxed 4:3, but ioj ate my fileshack file when they bought the site and I don't have a backup.
Basically the example had a layer on top with black bars at the top and bottom, only you couldn't see them in windowed mode because they were off the edge of the screen. Then if you go to fullscreen, there was a quick check to test the aspect ratio of the player's monitor based on their system's screen resolution. If it was a 4:3 screen, it resized the resolution accordingly, and the black bars would then be in view.
If you have a layer with a HUD you might need to manually scroll it down or something. And you might need to mess around a little with centering the view on the player, unless you have unbounded scrolling.