I'd like to display a notice (actually a popup using @Chris D's add-on) when a user views a thread which is locked, within specific nodes. I believe I could do this with the page criteria tab, but after playing around for an hour I can't seem to figure it out
Not very. The thread record would need to be available to the page container which isn't too difficult to do. Then the criteria checks would have the thread record available so you can read its closed state and act accordingly.