One last idea that I came up with is expanding the mirror function to allow mirroring diagonally, across either a guideline, or a diagonal drawn through two specified points. This would require two radio buttons, the first with a single field for indicating the guideline about which to mirror (see http://forum.high-logic.com:9080/t/position-to-guideline/3345/1 for guideline naming proposal), the second with four fields for indicating Point 1 X, Point 1 Y, Point 2 X, and Point 2 Y.
I think there are already several ways to accomplish this (mirror, rotate, move), and although your requested solution might make it easier it might make the program more complex and overwhelming for starters.