Okay so it looks like it’s simple to enable the clipping plane in the scene capture 2d component.
All you need to do is add something like this:
FPlane MirrorPlane = FPlane(FVector(0, 0, 0), FVector(0, 0, 1)); //Plane base, plane normal
SceneRenderer->Views[0].GlobalClippingPlane = MirrorPlane;
// Jitter can't be removed completely due to the clipping plane
// Also, this prevents the prefilter pass, which reads from jittered depth, from having to do special handling of it's depth-dependent input
SceneRenderer->Views[0].bAllowTemporalJitter = false;
In SceneCaptureRendering.cpp after about line 305.
Obviously a little bit more code would be needed to expose the plane parameters and add an optional boolean but my point is that this could have been added in like 5 minutes by someone who knows what is doing (I’m not 
lol) so can we please pleasee add this in the next 4.12 update?