I have never seen rock lath and plaster or wood lath and plaster any thicker than an inch in total wall projection. Are you sure it's 1 1/2 inches?
Anyway, another option is to cut nothing at the window and cut shims the size needed for where the trim will extend to the wall. These shims will be attached or slipped under the trim back edge where it meets to the drywall on its outside edges.
Anyway, another option is to cut nothing at the window and cut shims the size needed for where the trim will extend to the wall. These shims will be attached or slipped under the trim back edge where it meets to the drywall on its outside edges.