I'll measure the travel on the roller bridge later, but no way that you can get more than 2-3 millimeters. So if he needs around 4.5 mm on the bridge, I think he's out of luck with the roller bridge.

But maybe a combination of the adapter and the roller bridge? Or dowel the bridge holes and then redrill for the smaller posts off center? Then add the roller bridge on top? In any way, dowelling would mean messing with the finish, so no idea if it is out of question already.