This is a really great discussion. IMHO the best advice given has been to get out on the river, practice flipping boats and righting them. One of the awesome aspects of running rivers is the dynamic environment and no two flip situations are going to go exactly the same.
Not to jack the thread, but, as mentioned getting on top of the boat is going to play a huge part in success a lot of the time. As far as the belly strap goes I can see how this is a useful tool. I personally would never run a strap across the width (or length) of my boat, particularly as loose as the one shown in the video. I can see this being less of a hazard on the big flow runs, but regardless the opportunity for it to get snagged on a rock, wood, or human made debris is too great in my mind. I personally , and work for others that do the same, run small grab loops of webbing in the four corners of the boat through the floor lacing if it is covered by a flap and drain holes if it is a glued in floor. I am tall, but for those that are shorter another useful tool I have seen is a small patch with a d-ring that has a short tail of webbing glued to the center of the floor between the mentioned grab loops as most people struggle with getting the second hand on something solid towards the middle of the raft floor.
Lastly, I see a ton of boaters out there boating with no flip line around there waist and I often wonder what the heck they are going to do quickly for leverage to flip a boat. As seen in the video the flippies are searching for a few moments for a cam strap they had pre-rigged for the leverage. Again, this is a useful tool. However, the opportunities it creates for an entrapment outweigh the benefits IMHO. Flip line around the waist is where you know it is, a multi-purpose tool, that works great for leverage.