When I saw your reply in my mailbox I realised my mistake. I have read about what you want, do not remember where. One of the solutions was to render an animation, where the opacity of a black sheet in front of the camera was animated. But I think that is technically not the right way to create an hdri.

I think changing the light would mimic reality the best.
Using the master light might be good trick. The animation trick is still usefull. You could start your scene with the lights (and ambient light) all turned high, and end with it to almost none in 6 frames. Render the frames as sequenced photoshop docs and merge them.

But I have not tried it myself :)