It used to be that if you had no IRE spread, either all 0’s or all 1’s… indistinguishable from a blank sheet of paper.
What species of DNG are you working with? If you have any kind of metadata describing exposure index, maybe you can pump those numbers to see if there is anything down there in the inky abyss.
Then its all about noise suppression and banding. You are trying to scale a 100 foot cliff with a ten foot ladder.
jPo, CSI
“I always pass on free advice — its never of any use to me” Oscar Wilde.