You can build a VGA-to-YPbPr cable, the details are in this topic: viewtopic.php?f=101&t=31006
This will result a 'luminance' (brightness) signal on the Y plug, which is identical to monochrome composite video. You can feed this signal to your monitors, they will accept it, but there will be no colour (ideal for green Sanyo). Unfortunately only a few cores support this video mode. For your Commodore monitor it would be even better an 'S-Video' output, the Y signal is given, and a C (chroma/colour) instead of Pb or Pr, but it's not implemented (yet).