A simple 180 degree polarity switch on each track’s pre-mixer and/or post-mixer.
Also a simple phase rotator native effect where you can rotate each channel (left and right) in both directions (negative or positive). Maybe in this Phase Rotator native effect, you could also have the polarity switches for each channel (in addition to a main switch for the entire track mentioned above).
If I do not mixing up things, you already have a “polarity” / inversion switch in the gainer device.
+1 for a full degree rotator device, but I guess such a device is considered harmful for usual mixing engineers, due possible phasing in mono mixdown. Of course then you could also use it to fix mono problems, too. I made a little test here.
Yes you are absolutely right about the Gainer device. I would just love one on the post-mixer volume slider for the whole track so I don’t have to only use it on an insert device.
For the phase rotator, I guess I’m so used to mono compatibility during mixing that I figured everyone check for it. Ha ha 
I find that I need to use external plugins every time I mix when using reverb sends, there is always some tonal comb-filtering that is usually fixed with a slight rotator or a polarity flip. Would love to use a native effect instead. Would really flesh out Renoise.