Hmm. The thing I'm worried about with adding a GT style (smooth or toothed) adapter to the idler is that it changes the diameter of the bearing which will cause some inaccuracy. I wouldn't have to want to tweak the firmware steps more unless necessary.

I think I'm going to design two rings and will be press fit over the bearings which will hold the belt on the left and the right side. This way the belt still rides on the bearings in the center and it will just be held in place.