It's a bit more involved than my previous post might make it seem.
You can make a screenshot simply by copying the framebuffer to a file (e.g. cat /dev/fb0 > /usr/share/screenshot.raw). To copy the screenshot back into the framebuffer you do the reverse and do a screen update. For the screen update I made a slightly modified version of the "blank" executable.
Then it's a matter of modifying and creating /etc/init.d scripts (in particular the "blank" script) and changing the Irex firmware (the hardest part) to stop updating the screen on boot and shutdown and create the screenshot on shutdown.
The reason I copy the screenshot to the internal memory is because the sd-card isn't mounted yet when the init scripts run. I tried mounting the sd-card myself in an init script, but it failed for some reason.
If you're really interested and have a developer environment I can post the sources as soon as I've worked out the kinks and cleaned up the code a bit (there are still a few bugs, one of which is that it won't boot on the qemu