It seems that UE4 has the ability to render in stereo with the Oculus Rift without any kind of special programming. But I was wondering if there's any way to, say, tweak the stereoscopic effects. Let's say that you want some visual elements to appear on one eye, but not on another. Is that possible to do in UE4, or would you have to mess around with the Oculus plugin? This would be very useful to know.
Don't think it is possible, whether you mess with Oculus plugin or not. Oculus has nothing to do with rendering stereo, it is core UE4 functionality. You can render in stereo even w/o Oculus plugin, just use -emulatestereo command line option. Thus, you may want to ask this question to Epic Games guys (however, the answer still would be 'no', I assume).