Better all round to bite the bullet and imagine a poorly managed Double DC Gen fail scenario in which the voltage falls below the threshold for the DU's and there you have it - the ESIS alone to get you home.
If that's what you are trying to simulate then don't you need to plunge the cockpit into darkness, unplug the pilots headsets, knock out their autopilots and remove the ILS and Nav information as well? Which would probably be un-survivable if IMC.
If I understand it correctly, a 139 pilot will only need to fly an ILS on the ESIS in the event of a failure of either both AHRS or all 4 Display Units ? (All independent systems)
Would it not be better to introduce and teach the ESIS as a tertiary instrument for cross checking information from the primary and secondary systems and acting as an umpire in the event of conflicting information between them.
Of course, recurrent students should be made to fly on it just for ****s and giggles........