Model Viewer script tries to get model AABB and move camera away enough to see all content of scene.
You could replace content of setBestCameraPositionForModel with this:
this.reset(pc.Vec3.ZERO, this.entity.getPosition().length());
And it will pretty much focus center of scene and stay at same distance as you set it’s position.
I agree, it is not the easiest to hack around script 