You are right! Its a simple matter of combining the H and V sync into a single cable. Theoretically you need to XOR both syncs, but just joining them works according to some web site I was able to locate.

Things are never easy, it turns out that VGAs use a higher horizontal scan rate than most RGBS devices (30 something kHz vs 15.75 khz). So most video boards have to be reprogrammed at the register level to do this, which might be a no-no in an op sys such as WinXP. I don't know why everything I want to do requires hacking