The "ArcCenter" command always constrains the arc to the XY-plane, making it hard to put arcs where needed. For example the center of a box, and the top edge as the start and end would produce an arc that's about 45 degrees with the floor.

Is it possible to make it work in 3D, and have the "keep flat" as an option? And is threre a current workaround?

I can do almost the same thing with Circle3pt and some trimming, but it's not the same.

What about if I hack/edit this line in ArcCenter.js to a false value?
pointpicker.restrictToPlane( arcfactory.getInput(0).getValue(), true );