Ooohhhh, you’re trying to do compositing in realtime…well, that’s why is called compositing 
In realtime you need to find some tricks to make it work…one way would be to get rid of the plane and completely squash on the Z axis a copy of the mesh itself, then drive the X/Y scaling using the orientation values of the sun itself…it’s math, nothing too fancy, but I guess that you’ll have problems with the double faces of the mesh itself…a completely black material should do the trick, not sure if with different variations of black you’ll have compenetrating faces issues…but you can give it a try.
If you want to go with a masking option I suggest you to get a screen capture cube, get that into the material editor of the plane, and then try to mask out the results from the scene reflection, but I guess it’s quite hard to do that, but you can sure give it a try 