Duh. Solved the problem... I had a script in /etc/init.d that wasn't backgrounding properly, so the system wasn't actually booted all the way; as a result, the button wasn't working. Fixed that script, and the button now works as expected. Anyway, I confirmed the script in /etc/rc.button/reset does work, and you can replace the reboot line with whatever you want. I hope that helps someone.