The biggest advantage of sketching with a backlit sketchpad is the convenience of using it right at the eyepiece.  I use a converted 120 format slide viewer as the sketch pad, don't get the 35mm one, too small (Thanks to Chuck Dethloff for suggesting this).  The slide viewer uses a small lamp to backlight slides.  I replaced the lamp with a two color LED and added a slide potentiometer to adjust brightness.

This is a picture of my sketchpad. medalight.bmp (22678 bytes)
This is a schematic of the internal circuitry sketchpad schematic.gif (4536 bytes)
This is a link to where you can order the slide viewer Get Smart Products