This issue is fixed, and it required a replacement of the crank in addition to the front pulley. The belt wasn't the issue. My initial question was the wrong one.
I've accepted Nathan Knutson's answer above because it describes the troubleshooting approach that led me to the solution.
But for completeness for anyone maintaining their Spot Acme, Mitchell, a tech at Spot Bikes, answered me via email saying that the belt port should be tightened to 9-10 Nm.
Here's his full email response, which also describes a technique for troubleshooting the issue and general specs for belt tension on the Acme. I've emphasized the key ideas:
I'm sorry to hear that you are running into some issues with the belt drive setup on your Acme! I do hear what you're talking about in the video, but the interesting thing to me is that it appears to happen at the same point in the crank rotation on the front ring, which wouldn't correlate to the belt since the drive ratio isn't 1:1. If you have a white or silver marker, I would suggest putting a little dot/line on the belt and the ring where it starts making noise, and then rotate the cranks and see which one it lands on consistently when it makes the sound.
Another thing to consider - remove the rear wheel and pop the belt off of the front ring. Once it is off, rotate the cranks and keep an eye on the front ring to see if it hops or wobbles at all. I would assume that you've inspected the belt for damage already and that when the bike fell, there were no signs of it landing on the belt correct?
The torque spec on the frame removal piece to replace the belt is 9-10Nm. Theoretically, replacing the belt in a like-with-like scenario wouldn't automatically require you to re-tension it, but I would absolutely check the tension to ensure it is in the proper range before riding to avoid any potential issues. We normally use the Gates Carbon Drive app to measure tension, which should be 35-50Hz for the Acme.
Thanks,
— Mitchell