Dear Guillaume,
I was close to the area you visited around the 15 of April. This was no herping trip with friends and I only found, Uromastyx nigriventris, Agama impalearis, Trapelus ??, Tarentola boehmii, Acantodactylus ??, B. mauretanicus, but no snakes at all around Nkob. Anyway approximately 20 km towards the direction of Boumaine I found B. brongersmai in amplexus. in an nice valley. Here are some pictures.
Ulli