Found it! Yes, it was a vacuum line. That's a great picture, tyl604.
The orange-y line with the blue strip had gotten knocked off, and since I don't really know what it's supposed to look like, I didn't notice.
The shutoff vacuum solenoid is below that white thing, to the left of it in the photo. Applying a vacuum to that white thing does NOT shut off the engine, unsurpisingly.
What is that white vacuum thing on the top back of the injection pump?
Also, a gratuitous picture of my shut off valve.
Down low, on the back of the pump, below and obscured by the white thing with the green vacuum thingy plugged into it.