I thought I'd reply back with some updates on the scope triggering options. I tried each of the suggestions mentioned in this thread and had pretty good luck. I admittedly need a proper 100X probe to properly measure and tune my snubber circuit, but this was a useful exercise for triggering purposes.
I didn't have a chance yet to test out Circlotron's method with DC current, but I will.
Mask Test Triggering (MarkL)I created a waveform mask and set the acquisition to stop when the mask test fails, as well as automatically save the trace to memory. There are two disadvantages with this method, at least on my scope: 1) I need to use averaging mode because random peaking causes the test to fail, resulting in lost data due to the short duration of the back-EMF spike, and 2) the output is a trace capture, so I can't zoom in on the waveform after the fact. Even so, the technique was easy to set up and very repeatable.

Glitch Triggering (macboy)This method required some fiddling, but it did work well. The glitch settings were negative polarity with a duration around 1/2 the period, or ~8.3 ms for 60 Hz. I started at 16.7 ms and slowly reduced the duration until the scope stopped triggering. Then, when I cut the power, it would trigger right in the center of the screen. In this situation, I think the duration is largely dependent on the shape of the waveform and its timing at the trigger voltage level. I'm using a triac to drive the circuit, so the output isn't a perfect sine wave.
Back-EMF Triggering (David Hess)Turning on LF rejection and just triggering on the event itself worked reasonably well, although I'm definitely at the limit of my vertical range with only a 10X probe--the natural peaks and ringing from the LC input filter in my circuit interfered with the trigger, but I can see how this'd be a useful method.
Current Probe TriggeringA few posts up I mentioned about measuring the current with a clamp-on current probe... that'd work for triggering, too! I don't know why it didn't occur to me earlier. I just set the trigger threshold outside the steady state peak-to-peak value coming from the probe and triggered on the current spike at shutdown.
