I created a new wall style based on the one I am using, with a new very thin layer inside (1/32") using a neutral material/hatch. Then I use this layer near the exterior to create a new 4" endcap using the Endcap feature, pushing it past the other wall parts to create the trim. Then create a new Opening Endcap with this newly created end cap, setting it for the entire surround on the window (or opening), Next, associate it with all openings (as appropriate for whatever gets trim), reset the wall endcap back to normal, turn off endcaps in plan view, turned on in elevations. Works well for arcs, and I've also created brick sills and soldier courses with it (though not for arcs)
Bonus is that you don't have to create a new window everytime you use it. Just place opening of any type in wall, and instant trim!
Mike T.
Jacksonville Fl.