Never thought my first forum post on DZ would be about jumping with dolphins... Anyway recently I did a clear & pull on the back of an inflatable dolphin and riding it down on the back. I exited at 6000 feet and we allready lost some weight earlier.. So there was just one other jumper left behind me and I had all the space.. I was able to get out facing the prop hugging the dolphin with it's tail clamped between my legs and choking the neck of the dolphin with my left arm (no worries it's not a living animal). The pilot was flying with almost zero prop and still it was one hell of a fight in the door. Somehow I was able to make a sort of stable exit and pull my pilot.. On opening I got a bit on the side because the dolphin got air.. While flying down the density is increasing which makes the dolphin deflates and this makes it harder to clamp the dolphin. So looking at your plan... I would say that making a dive exit the way you describe it...impossible unless it's a tiny dolphin. The dolphin will get air and unless you have strong arms and are some tunnel ninja you will be unable to fall stable. Please stay safe!