Hey, I like your innovative thinking. Maybe Teflon treat the rolling surface to wear on the belt a bit better? On some race application's I have seen timing belt idlers that I would swear were just a skateboard wheel machined down to fit. Good quality aluminum should wear on these belts fine IMO. I know the Opel engines in Europe these engines are copies of do have metal idlers in production, not sure why Korea went with the plastic stuff, other than to shave cost.