I don't think a big e-ink display is that much more useful than a small one, unless you really want to read books on it, but then it's going to be too expensive if we want to do it right (high res, decent refresh rate, enough grayscales for anti-aliasing etc).
Also I think it's a bit wasteful to put a display in the back of the lid because then you can never use it while using the real screen (except maybe for 2-player games?).
Something like what I linked earlier (
http://www.sharpmemorylcd.com/1-17-inch-memory-lcd.html), 184 x 38 pixels, enough for 20x4 characters or so, would be perfect to build into the front of the base. We could write a library to make it easy to use in applications and a simple program to use it from scripts, e.g. just do
/usr/pyra/scripts/set_eink.sh path/to/some_black_and_white_184x38_image.png
and the image changes, or write a line of output by doing
echo "Hello world!" > /dev/pyra-eink
It doesn't have to be more complicated than something like that.