ups... don't saw this section. ;)
Merci
Actually, you can envelope entire scene... Just do not use it to illuminate scene. You can have exactly what you have now, just use CoronaConstantMTL, instead of emissive CoronaMTL. It will give you some performance boost.
To be honest, Keymaster misunderstood the difference between environment map and background plates...
CoronaConstantMTL is material exactly for this purpose. Common workflow is to first light your scene with daylight or HDRI, and then put geometry behind windows and ConstantMTL on it. ConstantMTL is optimized to affect scene lighting as little as possible, while still provide directly visible background as well as reflections.