FYI: the way I wanted to do the boulder was to have a regular physics sphere and move a little gate blocking it from rolling during the sequence. Which worked fine except the default behavior for sequences is to have everything reset to their original values once complete (“Restore State”). To override this, you have to do two things: 1) right-click in the track of the thing to want to persist (i.e. the transform of my gate), click Properties and change “When Finished” to “Keep State”; and 2) in the trigger where we tell the sequence to play, unclick the box in “Settings Restore State”.
Also note that the position of my gate in the level depended on what frame I was on in the sequence editor, so I had to make sure I was on frame 1 when I started the level or the boulder would start rolling immediately.