I finally found instructions at
http://pocketpcman.com/CDs/hpCD/560series/560.htm . I printed them out, finally found a T6 Torx driver (hard to find!), and got to work.
The Jornada I picked up for parts was from a friend, who said it just stopped working but before that was working great, including its screen. He suggested I first try taking apart his Jornada and putting it back together, in case it was just a loose screw or something. I did that and it worked perfectly! So I'm now using his repaired model and have my old one for parts.
My friend's Jornada was in much better condition than mine. He always carried it in a case on his belt, while I carried mine in my pants pocket. I dropped mine a few times and it was pretty banged up.