I see no point in having separate stretching options when dealing with square pixels. The idea with a checkbox is to avoid mistakes. If you put a 16:9 1920 x 1080 picture in DVD-o-matic and select FLAT as output you will get a slight horizontal stretch. It can be hard to see if you are tired and in a hurry. (been there, done that..)
OK, I agree about the "Magnus list" (with NTSC added). It would be far too long. Zoom (scale), square pixels on/off, offset and crop would satisfy me. Maybe combined with a before/after comparison view?
268 pixels! On what screen size? 0,27K :-)