Yes it is possible, atleast with the SDL2 build.
I am using "Keep desktop resolution in fullscreen" and SDL2 options "Use linear scaling" to get a more smoothed picture for the rediciulous large resolution compared to the ST output. This gives me a nice centered 4:3 ST screen with optional ST borders and black borders left and right of the emulated ST screen on a 16:9 TFT.
Simon Sunnyboy/Paradize - http://paradize.atari.org/Stay cool, stay Atari!
1x2600jr, 1x1040STFm, 1x1040STE 4MB+TOS2.06+SatanDisk, 1xF030 14MB+FPU+NetUS-Bee