Another option for the type of BOB you are talking about is a vest. It would conceal fairly well under a jacket, shirt or sweater.
Think of fishing vests, they have pockets and pouches for everything the angler needs. You would probably have to design it yourself but you could have a place for everything just the way you want it. It could include a built in holster. Plus a vest distributes weight more comfortably than a pack.
I love this vest! It’s been my walking in the field office, camera bag plus. Just plain outstanding. If I forget in which pocket I’ve placed something it’s a five minute pat down >;)
They have similar summer weight ones as well.
http://www.duluthtrading.com/store/clothes-footwear/mens/vests/86072.aspx?feature=product_5#