The classic weight belt is pretty much independent of the BCD, so it should not matter in which order you put it on as far as the practical use is concerned. It should end up in the same place relative to the BCD either way. If you put it on first and it is trapped under the BCD, the adjustment takes only a few seconds.
When I use a weight belt, it always goes on first because it is a lot easier to put it on before the BCD is there to get in the way.
1+
Weight belt on first for me, for both a standard jacket-style BC and for a BP/W with Hogarthian-style harness.
The BC cummerbund or BP/W waist strap will for most folks typically ride at approximately navel-level, a weight belt almost always rides a couple inches
below that level at the crest of the pelvis, and as John says, can be adjusted easily enough if it does not. The cummerbund should not cover it if the BC is sized properly for the diver.
With a BP/W, wearing the weightbelt under the crotch strap requires you to release the harness waist strap first, let it fall away, then release the weaightbelt.... so it doubles my time to drop weights in an emergency from 3 seconds to 6 seconds
.... not a significant problem, and the weightbelt is more comfortable and secure
under the BP/W waist strap (but is certainly "ok" above it if that is what you prefer).
Best wishes.
Edit: And for me, putting the weight belt on first prevents me from entering the water "sans weights"
A bit embarrassing to not be able to sink and discover the weightbelt is back at the car after a long swim out... don't ask me how I know this....