Have you fiugured this out on your own?
I just played around a bit and found a method, but it's not exactly intuitive. Select the object(s), then switch to the Rotate tool. The center of rotation will be defined as one of the transformation handles, but you can drag it to anywhere you like, so move it to the point that you want as the center of your mirror. Hold down the Alt/Opt key, and click the reflect button to make a mirrored copy. Without the modifier key the original objects will mirror their position around the repositioned reference.