How can I cause my oscilloscope to trig on interruption of AC voltage?
In the trigger menu (right side below the trigger level knob: "menu") you can, if you have it (I think Timeout might be in the upgrades? You could hack it if you don't have it anyway) you can select Timeout.

You then set slope up and down and a timeout of 75% of the sine period, in the case of 50Hz AC that would be 15ms. It will then trigger when no slope has been detected after 15ms. (So you will get 15ms of samples of 0V, but if you adjust the trigger to the right you can move a lot of signal into your sample depth.