From your link: "Proper belt tension is essential for optimum operation of the Gates Carbon Drive System. Lack of belt tension can lead to tooth jump or “skipping,” when the teeth of the belt slide over the teeth of the rear sprocket. Too much tension can damage the bearings within the rear hub, can cause the system to drag, and can increase the wear of your drive system."

I think we can agree that there's no danger of too much tension in my setup. And the snubber wheel prevents tooth skipping. So where's the problem?

Bear in mind that when you set the tension in a 'normal' Gates set up, you're only tensioning the belt at rest. You'll exert forces far, far in excess of those stated on the upper part of the belt under load.