Scanned the responses and didn't see this mentioned. Wearing one weight on the belt, placed at the rear center of the belt is going to be uncomfortable, somewhat unstable, more easily trapped by the BC and a bad idea.
A better idea is (as mentioned by other - get a rubber belt) and place two pieces of lead on the belt. You can purchase weights down to the 1 lb size, so you should be able to accomplish this. Wearing small weights on a rubber belt with the weight located on your hip crest (or preferably just forward of it), will place ballast more below you improving stability, but it will also be more comfortable and won't have any tendency to spin or rotate around your waist. The rubber belt will cling to your body and allow you to comfortably wear it just below your waist harness straps NOT under it.
I wear my weight belt under the crotch strap. It will be slower to ditch, but much easier to put on. If you are wearing a belt with less than 3-4 lbs on it, ditching it super fast is probably not a safety issue anyway - more convenience. Try wearing it under harness and crotch strap and see how it feels. After several times of dressing and undressing, you will gain confidence that you will be able to open the belt and draw it away from your body when worn in that position.