Sorry, this digs back a bit. There are two problems with extra lift in lieu of ditchable weight. First, it is hard for your buddy to cure your negative buoyancy problem at depth. With weights, your buddy can just ditch your weight and up you go. Similarly, if you are unconscious on the surface your buddy can ditch your weight and keep you there. This is far easier than inflating your BC.
Second, the situation in which you envision having to become quickly positively buoyant is when your BC is not working. Your suit is compressed and you pull the corrugated hose off your BC. Oops. Now what? It is possible but extremely difficult to put air in the BC. It is possible but extremely difficult to swim your weights up to the surface. It is possible that your buddy will think to add buoyancy to his own rig to help you surface. But a much easier and more direct solution is to ditch a few pounds and pop up.
I'm not getting a clear picture of the procedure you used to don your belt at the surface. In OW, my instructor had us do something really simple. Reg in the mouth, we held the buckle. Then we lay on our back with the buckle against our belly (so the belt was already wrapped partially around our body, and was hanging underneath us). Then, still horizontal, we rolled over. Now the belt is lying comfortably in position across the back, with the ends on either side and the buckle still comfortably gripped. Grab the other end, and thread it through the buckle. Now you can go vertical and adjust if you want, or roll over, or whatever.
