I agree with Trevor about aging in bulk instead of in individual
bottles. The wine will be more consistient and will advance it's age
more completely if you let it happen in the larger carboy.

That having been said, properly dissolved campden (or almost any
chemical) will, with a stir, become completely dispersed in the liquid
in a very short time. If you were to stir it the campden first, then
get everything together for bottling (which you won't do, because
you're going to bulk age, correct?), the campden will be well dispersed
by the time you start filling.

Rob