After more than a month without a lockup (sitting on the laptop cooling pad), I finally had another lockup a couple days ago. Light was solid Blue. Didn't respond to anything, including pressing the reset button. Powering it down and back up was the only thing that worked. Nothing recorded while it was locked up.
Fortunately, I was home this time, so I was able to reboot it and had less than 12 hours of downtime (it took a while to notice it).
I'm going to look into hardware to be able to reboot it remotely, in case it happens when I'm out of town again. Might even look into some sort of automated solution (like an IFTTT recipe) to detect if it locks up, reboot it, and send me a text.