I know there's a mirror plugin (Toli's effects) out there, but noticed it's from 2007 and now errors on loading. I created an up to date one to mirror a selection horizontally, vertically or both with CodeLab (Took less than 15 minutes! Pretty cool plugin!).
Mirroring the selection is done by clicking Adjustments at the top menu, then Mirror selection. A new window should pop up offering your options and previewing in realtime.
Edit: On trying again, noticed I forgot to copy a DLL from toli's effects, appears his plugins still work. This post can be deleted if anyone wants it to be.