I have been in 2005 in Bahr el Ghazal in Therapeutic Feeding Centers. I get all permission with help of humanitarian organization Action Against Hunger USA
is responsible for south of Sudan and French AAH
take north including Darfur. You could try to talk with French Action Against Hunger.
There is a visa if you going from the north and there is a SPLM
travel permit if you travel from south(stamped by SRRC
“Sudan Relief And Rehabilitation Commission” with office in Nairobi). Difficult to say what is better for you. If someone from the north will get you with SPLA
papers you can have a trouble. But I was there some time ago.
Before I take my camera to hand I had to wait for the meeting with authorities. But after that the only problem was that everybody wanted to be on a photo.
I also tried in Kakuma refuge camp in Kenya. I traveled there with no papers and they didn’t let me to take any pictures.
Good luck and I hope you get great pictures.