What I was thinking with respect to oversampling is that the OSSC Pro could oversample at a rate matching the console’s subpixel clock and then scale it down horizontally with a box filter to say, 1440 pixels. For instance, the PlayStation2 has a 67.5 MHz subpixel clock (135 MHz in progressive mode). Sampling at that rate would catch all its output resolutions pixel perfect with one setting. It might be madness to do that compared to say, 27 or 54 MHz and let the input low-pass filter do blending of pixel boundaries (at 67.5 MHz the active area would be 3,200 pixels wide). But it would be fun to see it tried out.
Regarding the vertical stretch for PAL, this is where I hit a snag with the OSSC and my TV. The TV has limited compatibility with non-standard resolutions, especiall at 50 Hz. There I’m limited to Line2x mode, and the TV assumes the start and end of active video to comply with standard 576p timings. Trying to adjust the active area or backporch doesn’t affect how the TV handles the picture (increasing the backporch simply blanks part of the picture). Having a pixel aspect ratio control in the OSSC Pro with provision for PAL correction would solve this type of problem. I imagine compatibility is much worse with 50 Hz than 60 Hz even on newer TVs.