Basicaly you are going to have a geared bike.... why go through the cost of internaly geared hub? It would work but I can't see a reason why to do it, I've never used a internaly geared hub, but I don't think they would work well for off-road because of the abuse it would take.

Why not just get a White Industries ecentric hub if you don't want to use a tensioner, or if you select right gearing you can actualy get away without a tensioner this limits your choices ALOT though.