Not exactly what you are looking for but: Select the area you would like to clone, copy it, paste it into a new layer, then flip it horizontally and place it where you need it. Does that help solve your problem?
If you have two images open at once, you can clone from one into the other; didya know that? So just dupe the image, flip it, and clone over to the original as desired. I suggest doing the cloning on a blank layer, so you can erase away selectively, or toss it and begin anew if it doesn't look right.
Depending on what you're cloning you may be able to simply set the degree of the clone source (i.e. windows/clone source then enter say 180 for the degree).
Word to the wise: While all these suggestions will work, none are legal in either Basic or Advanced editing. So if you're thinking of doing this to a challenge entry, you'll want to think twice. For art, gallery, stock, commercial and such ... well, clone away! They'll work great!
it was fo a side challenge, I wanted a broken window glass pane flipped at certain angles. It has been an interesting experiment, not one I have mastered though!
