Any way to trigger JavaScript at a particular video timestamp?

There might be more than one way to do this but here is what I would do. Add a blank slide after the video you already have in your course where you wish to run Javascript at a certain point in the video. Return to the slide with the video and click on Edit Video Timeing... in your Properties Inspector. Slide the scrollbar to the end of the video and drag the slide transition marker to the point in the video timeline where you wish to run JavaScript. Then click on the insert slide icon to place the remainder of the video on the second slide. Click OK. You will now see that your video spans the two slides. Select the second slide and set the On Enter Action to Execute Javascript. Place your script in the script window and decide if you want the video to pause and wait or continue playing.