You can't change the background itself, but you can change some of the built in icons and text by editing the firmware. I successfully used
iPodIcons to change the "do not disconnect" icon of my iPod photo to a picture of my dog's face (then it was stolen, so some bastard is out there enjoying my puppy's cuteness
). Doesn't look like it's been updated for 5th gen, but who knows, maybe it'll still work? I seem to recall this program was smart enough to work with any iPod firmware at the time. Might be worth checking into.