When using a weight belt with my BPW, I wear it beneath my crotch strap. I don't really worry about removing it in an emergency as I can easily swim up with the weight. If I was unconscious, a couple burps on my inflator would make be plenty buoyant. Just don't use too much weight.
Usually, however, I use little 5 lb. pockets on the waist belt pushed back close to the plate. In addition, I have a couple 5lb pockets on the top cam band for trim weight. The weights on the cam straps can be a pain in the ass to finagle when the tank is clamped into that omega looking thing to keep them from tipping over, especially if you have them threaded onto the bottom band. I've tried to mount small 1 or 2 lb. weights to the shoulder straps where they come out of the backplate (behind my head, close to the first stage) but have not had any luck.
Ultimately the best place for your weights is the backplate itself. Your lungs are your buoyancy device when it comes to trim and vertical adjustment in the water column (not your BCD); then the weight should be centered over your lungs.