Specifically i'm asking if there is a way to keep opengl ouput always scaling the image to a fixed (desktop) resolution, not only the first time.
Sure i can always use surface, but that distorts the game image on my 'widescreen' (not really) monitor because it refuses to introduce black bars! In effect i've found that only games that can set their own resolution to something with the same vertical resolution as your native desktop are the only ones that look good with fullresolution=original and output=surface. And by only ones i mean system shock with the mouse mod and settlers 2, which are the ones i found with a 786 vertical res mode.
If surface didn't insist in stretching the image without respecting aspect ratio, fullresolution=original and output=surface could be the best.