I can see where they wouldn't want to do an NPC for ammo MANUFACTURE as this is a player income source. That being said, the obvious answer is an in-game buletin board service allowing players to commission ammo from a crafter for hire. The fact is, the bulletin board could also have other uses, like planning for an event and the like as well, hiring a mercenary crew to help with a certain battle, etc. This would strictly be posting only, for security you would want to have all dealings via message/tell and deliveries via DB and/or direct trade UNLESS SE wants to do something crazy hard like an in-game escrow service. Face it, no one would want to trade 500k in crafting materials only to see the guy log and disappear, then AH the mats on their mule. Problem is, I suggested an in-game BBS once before and I seem to recall I got an answer shooting it down, but also saying these forums were not an appropriate place either to do the advertising.
UPDATE: I forgot the "wanted" bazaar feature on ffxiah, you could work a commission there if you wanted to, I've used it a couple times for items that never stay on AH, no fees. not sure how you would message in that it's a commission, not a sale.
Hm... sounds like a business opportunity here...