Hi Mac,
Sadly I tried this prior to posting the suggestion. Unfortunately the problem with using gain in the post process is that affects all lights, placed as well as ambient.
So if you reduce the gain it gets really dark. There is no light at all as placed lights, candles, spots and the sky whatever are also reduced in brightness.
Here’s an example. normal gain 1st.
Now 0.25 gain. The placed lantern, and the daylight sky have all darkened. I guess this is the expected behavior as gain is acting like the gamma setting in other software?
If we could have a lut/darkness volume placed lights and the sky/ambient light outside the volume would be unaffected and only the ambient light inside the volume would be reduced, or indeed increased.
I confess I do not know if this is possible as I have no experiance of the unreal engine other than fiddling with an early version of its map editor.
Edited 'cos my typing is rubbish 