Just thinking out loud here ... Imagine, if I have a HDRI with clouds and a sun .. and a geometry which represents a city shot from a helicopter.
How do we get cloud shadow area's ?
Is it possible to add something like an opacity map to the light coming from the HDRI ? Or wouldn't that work ?
What your talking about is a gobo, but from a HDRI... I've never seen that and would stick my neck out and say it hasn't been done yet.
But you can map the opacity for an area light source which means you can simulate the shadowing of out of frame objects and efffectively shape the light.
That would get you the effect your after
J
Maybe the best way is to just put a plane just outside camera view and add a cloudy opacity map to that ... I should give that a test.
I doubt that will work - but using an opacity map on a light source definitely will
Anyway good luck with it