Ok Jason, I'm not a fisherman by any means, so take my suggestions as those coming from a mere passerby. What if you streamlined your fly fishing pack down even more by eliminating the need to accommodate a separate Morrell style fly box? Seems like carrying a case within a "case" or pack seems redundant in this particular instance since you're developing a streamlined UL fly pack for lightweight western fly and Tenkara style trips where you'd only be taking a very small quantity/variety of flies anyways. I'm thinking a small square piece of 3/8 or 1/2" evazote pad maybe a few inches in size(ex. GG thinlight) heat scored and folded over on its self in 1/2 could make a simpler and lighter way of storing your flies. You could possibly use Lycra mesh or a mildly elastic cord to secure the evazote pad when stored inside the pack, again, kind of like GG does with their pad holders on their packs. It could possibly be a flip down "work bench" style pack design too, just with a simple piece of evazote foam pad as the fly holder. Modular in design, the user could simply replace the small piece of foam pad once it become worn out inexpensively. Would this work? Anyways, just a layman's idea.
One other thought, what if you incorporated a simple stretch mesh pocket for a small point and shoot camera? This might add some bulk to the wearer with a camera stored since it's on your neck but would be conveniently close to other essentials needed for a UL day at the stream.
Also, sorry, I'm rambling a little. Are you going to use cord to wear around the neck? What if you incorporated x2 sewn-in Linelocs at the top of the pack and used 3mm cord? This could be easier to micro adjust to each user than a single cordlock behind the neck. Just a thought.
Last idea, since we haven't seen what you're pack will feature or looks like, if you're incorporating a closure system using quick release buckles, perhaps you could use a whistle integrated buckle somewhere on the pack? Or is this getting too feature rich?
Looking forward to seeing the pack by the way.