MooseMike,
I sent you a PM earlier, but I would like to expand on this a bit for you... Many outfitters do not own or manage most of the land they hunt. It's for that main reason they do not specifically offer culling/management opportunities. Additionally, there is not a great amount of profit to be made by offering cull hunts.
That being said, there are many reputabable outfitters who do own/manage their concessions that would be happy to offer you culling opportunities during your hunt. We are going on our 5th RSA hunt this August, and we have honestly run out of room for any addtional large trophy animals. Most of our hunting these days consists of cull hunting broken horn males, females, small unusual African species, and all the varmints we have arrows for... Cull animals still make wonderful backskins for rugs and throws. We are proof that you can still enjoy all of the experiences that Africa offers, get to shoot lots of game, and not have to spend a great deal of money in the process.
Inquire with your potential outfitter as to how much if any culling opportunity that will be able to offer you...