Materials: Control UV Mirroring through Parameter

So I am working on scene that has a lot of flags and banners of sports teams. I intend to make one master flag material with a Texture Parameter, and then create material instances of all the various flags I need. But I have a conundrum when it comes to the backside of the flag material.

Some flags, take the US flag for example, are a mirror image of the front face, on the back side. In my case I have many flags that have sports team logos and/or text- and they can not be reversed on the opposite side. So, basically I need some type of parameter in the material instance that will let me toggle the UV mirroring of the back face.

Could someone point out the correct way to set this up?