I wrote too soon; the problem's still there. The only way I can get the tablet to dim immediately (or after one second) after a long session on mplayer (at least the length of a Futurama feature movie), is by going into Control Panel, change the settings of screen to something else, lock screen and keys and wait for the new setting to run its course (luckily there's a 30 second setting now), unlock it, go back into Control Panel and put the settings back to 5 minutes each. Only then will my tablet dim the way it should.

Mplayer does not appear to hang; at least, neither ps nor top show it running after exiting.