Nice work, always glad to see more modern features in Urho3D 's rendering.
Why can't the pre-filtering be done at scene load time? It doesn't need to be super accurate. Heck you could almost just generate mip maps from the environment map, put on trilinear filtering, and call it a day. You could add a 1 pixel pre-pass blur to each mip to hide the pixels, or do a quick 3-tap blur filter in the final shader. Then mix the lower mips into the higher ones to better match a wide specular lobe.
I've seen the preprocessor solution used in other code, and I think it works pretty well.