Unfortunately HDR is supposed to "just work" in Unreal, however when you do use it, it.. doesn't exactly look good, and that is the end of the documentation. It's one of those things we would like to get working further down the line, but we had to prioritise other work on the game.
Thank you for the response and insight.
Absolutely, the game itself needs to be great before thinking about HDR.
I do believe the game is already great, with only some lingering bugs. Congratulations to you and your team, you gave me something no one else does (an amazing isometric single player experience, with great visuals, great gameplay flow-feedback, an amazing WORLD).
I wish Microsoft-Bethesda could give you the Fallout IP...