I have a dead scandoubler for it. it does not display anything, and (IIRC) it wouldn't even pass through the RTG modes (jumpers set correctly). but the 'main' output works. the VGA connector on SD is a bit 'chopped'. so, I was only hoping it was a simple connection problem and wanted to trace/check the RGB lines.
BTW, that idea you had could still be useful for people with only that scandoubler (if they exist!) and some other gfx card.