Clarification
The problem is not with the zipper; the Pilot uses only a single zipper on the bottom of the rear outside pocket. Opening that zipper forms a pass-through for attachment to a wheeled bag. No problem with closure there. The top opening of that same pocket, however, has no method of closure as it is meant to accommodate magazines, newspapers, and other things that need to be stashed/accessed quickly. Mine sags terribly, even when empty. I have lost a couple of things (non-important) out of it on each of its three trips.
I contacted the folks in Seattle and was told that is one of the traits of the halcyon fabric. They also told me that alteration of the bag (i.e. adding snaps / hook-and-loop) would void the warranty. I guess I just made the wrong choice as I was focusing more on weight rather than structure. I'll probably be going the hook-and-loop route in spite of losing the warranty as I am not pleased with either the appearance or functionality of the bag in it's current set-up.
If you are looking for function coupled with crisp lines, I would recommend the ballistic nylon version.