If I add an effect to a track, I can put envelopes on the track that control any of a number of settings on the effect. But is there a way to trigger an effect to be automatically enabled or disabled entirely during playback? None of the options in the menu for an envelope seems to be "turn the effect on/off completely".

right click on the bypass button. you might need to click on the plug in itself to open it up, then right click on bypass button.
there should be a drop down for on/off automation right there. similar to the M/S button automation.