JS Stereo Volume/Pan/Polarity control will do it. It's a bit more complex than you need, but I guess gives you more utilitarian tools for adjusting the stereo image.
A JS that just inverts whatever comes through it - on as many channels as you want - would be preset simple to write. You'd control which channels are affected with the plugin routing pins.
I think it would be super neat if we had this right at the Plugin pins, and I think I'll go make a feature request about it. (Edit -
So I did)