Weight pockets on your harness / cam bands, best solution.
Diving a weight belt with BP/W is not only uncomfortable but also unsafe, if you have to dump weights you will struggle getting to it and it can get caught on your harness.
If you need a lot of weight try getting a heavier back plate or a P weight (slots on the backplate) so it will reduce the amount you will have to carry on your harness.
Safe diving
This is not true. I've been diving a BPW for nearly 15 years and have always worn the belt under the crotch strap. If positioned correctly, weights and harness, it is not uncomfortable nor dangerous when you are properly weighted. If properly weighted you only need enough weight on the belt to be neutral. Not heavy. Weight distribution is the key and putting lead on several systems, using steel cylinders, and some common sense is called for. Weights on the cam bands, bolted to the plate, on the harness, etc. as well as the belt.
If you need to dump weight to get slightly positive a couple of velcro pockets on the belt to hold a couple of pounds is all you need. Once on the surface, then you can drop the belt quickly by undoing the buckle and harness buckle in one smooth motion with practice.
The waist belt will come undone, the crotch strap drops away, and the weight belt goes. I teach rescue classes using my belt under the crotch strap. In that case, you simply slice the harness and crotch strap like I have my students do and drop the belt.
Not being able to ditch a belt efficiently in any well-planned gear set up is a skills and training issue. Not a fault of the belt.
I have never struggled with getting to the belt in a BPW. The only time it was somewhat of an issue was in a jacket BC that was two sizes too big on me. Practicing with using any BC of any size to get neutral and in trim within 5 minutes in the water just for fun. The overly large cummerbund and sloppy fit covered the belt and it took an extra 30 seconds to get it.
I use a MAKO freediving rubber belt and it's one of the smoothest operating pieces of gear out there.