Alpha 5 uploaded. The patch has been rewritten for the surface output type, supports windowed mode and different machine types.
While implementing support for all machine types I was disappointed to find out that some them, such as vgaonly, seem to apply internal pre-scaling that prevents my patch to approximate aspect ratio effectively, e.g. vgaonly interprets a 320x200 game as a 640x400 one, depriving my algorithm of the necessary elbowroom for pixel-perfect manipulations. I consider this a flaw and think that the emulation of the video subsystem should be rewritten in such a way as to provide a point where the original raw image data is available. The patch takes it from RENDER_Reset(), where the pixel array is already modified.
I had DOSBox crash once during testing but couldn't repeat it. If the patched version should crash for you, please let me know in what circumstances it happened.