[DonRotolo]

My concern is the moving tensioner. Any time the load on the belt gets high enough, the whole belt will move the movable tensioner, and there will be slack somewhere in the belt.

Somewhat like chains, enough slack and the belt will slip over the sprockets. Very UNlike chain, teeth will pop off the belt when this happens.

Don't ask how we know this. In Aim High, we used a belt that was subject to high and variable loads, and had a spring-loaded tensioner. You do the math.