It appears that doublescan modes show up with twice the refresh rate that they
should in xrandr.  For instance, I have the following mode in my xorg.conf

  Modeline "320x200" 17.00 320 336 360 400 200 203 209 212 -HSync +VSync DoubleScan

and in the xrandr output, we see the following:

  Screen 1: minimum 320 x 200, current 1600 x 1200, maximum 4096 x 4096
  VGA-1 connected 1600x1200+0+0 (normal left inverted right x axis y axis) 352mm x 264mm
     [snip irrelevant modes]
     320x200       200.5

While a vertical refresh rate of 200Hz is well beyond the capabilities of my
display device, the mode nevertheless works properly: the on-screen display of
the monitor shows that it is correctly running at ~100Hz.