Offroading... the usual problem with this is that the chain hits the chain stays on the frame. I see there are 2 ways of resolving this, design a frame that the chain would not hit the chain stays, or use an internal gear hub, so a derailer is not required. what do you think?
I find the best way to avoid the chain slapping is to use a road rear mech. They tend to use a tighter spring and with them being a shorter lever don't allow the weight of the chain to make them recoil. I usually use a shimano ultegra or tiagra or even a sora if I need to go cheap. If that is not an option I will build up a layer of old inner tube at the contact point on the frame.