You will need something pulling the top and bottom together. Otherwise you will get a big skippyball once it is inflated. Because a ball is the shape with the largest volume and the most equal pressure for any given surface area an object will always try to take this shape unless it is forced into another shape (by baffles for example). Have you ever tried lying on top of a round surface? You can't, you'll fall of.
Like you mentioned, you can counter this by underinflating (= not enough pressure from the inside to fully expand into a ball) and applying more pressure on one side. The problem with that is that you can't apply equal pressure by lying on top of a balloon/mat. For starters, all the places where you're not lying will have less pressure on them than the places where you are lying. This means all the air will flow to the sides and your body will be (almost) on the ground. If you are not lying perfectly in the middle of the mat, more air will flow to one side where there is less pressure. This will mean one side of your mat will be higher than the other, causing you to roll to the lower side. This in turn means that more pressure is applied to the lower side, causing more air to flow to the higher side where there is less pressure, causing you to roll of even further. You would have to constantly correct your position and maintain very good balance. That's hard to do when you're asleep.
When you are applying pressure on your water bladder with your hand you are already making a lot of corrections for balance. You may not notice it, but you are. Try putting a flat plank with some extra weight on top of you water bladder. Once you found a balance, try moving the weight about 2 cm. It will most likely roll off.
Suppose you manage to find a good balance and you don't move during your sleep. I still don't think you'll sleep very comfortably. The reason for this is that your upper body is a lot heavier than your lower body. This means pressure is not applied equally. Your upper body will most likely be on the floor while your legs are high above the ground. Your head is also lighter than your body, so that will be higher as well (like on a very thick pillow). Prepare for numb legs and probably a sore neck in the morning. You can partially counter this by only making a half lenght pad, but then you'll still have to perform the balancing act described above.
