I've got a MOLLE/PALS adaptor panel for the Tac-Vest that will velcro/strap on to the sides of the Tac-Vest that should be online in the next week. There's as well as a pouch similar in concept to the Berry Pouch, which holds 2 mags that are secured via bungee retention, allowing the user to double his mag capacity on the tac-vest. One for the 521 is also being worked on.
C9 barrel bags are a bit more problematic, due to the melt/flame retardant material used as the lining, in that asbestos which is used in some of the older bags is extremely toxic to work with, and other materials such as Kevlar or Nomex are extremely expensive. There gets to be a point where in doing something like a C9 or C6 spare barrel bag where the cost would push such a piece of kit well above $100, not something that alot of people are going to spend money on.