It's not actually fog, but for medical visualization I am rendering a patient under simulated x-ray, and treating soft tissue as volumetric fog is quite appropriate - each organ a simple hollow hull representing a region of volumetric fog.
I found this on the topic which sounds pretty interesting - basically you render front-facing polys to one buffer and back-facing to another and work out the distance between them to give fog depth. But what if you have multiple bodies, and they don't have the same fog density? The link mentions you have to consider this, but didn't as far as I could see offer an answer.
Any suggestions? Either on this technique or alternative approaches? I'm happy to hear about fancy hi-tech solutions but I am targeting iPads which means I'm limited to GLSLES 2.0 and not uber-powerful GPUs.