Darren and Mark, thanks for the read and suggestions! If keeping it all natural fiber hadn't been key, lycra binding could be a great idea. Since completing the vest I've decided that my next iterations will simply fold the exposed edges under, then sew closed right on the edge. Western and FF seam (sorry) to do this.
The finished weight is high compared to what we're used to seeing. However, I have a plain jane Marmot down vest that weighs around a pound. Once you've gone to the lightest-option fabric, as in the 450g Momentum version, there's only two ways of cutting weight. First: Use less fabric--ie, lose the hood, the muff pocket, and the drop tail. Except those were in large part the point of making the vest for me. That said, if you just wanted a basic baffled down vest those changes would be easy (even simpler!). Second: Use less down. I designed this thinking of being completely stationary in 20*F or so weather. My personal experiences with different densities of down fill also mirror the findings of Richard Nisley (posts on this site), in that a greater density of down in a garment lends to a warmer garment. Cold spots in down bags and garments is kind of a pet peeve of mine. But if you didn't want that much warmth, just use less down!
Regarding the baffling, I think it was one of the best moves I made. Really warm. I found one cold spot in the vest this winter--the back of the neck. Although I baffled through that area, you might have noticed the baffling material separating panels and hood. There's no down in between those; I assumed loft would fill the space. Wrong! When a cold breeze blows, the back of my neck is the only cold spot now. I'm pretty well convinced all my future endeavors like this will also be baffled.
I've made a few odds and ends lately, such as a 2.65 ounce pair of down socks and a 1.45 ounce muff (to replace gloves). With the socks I did find it quite useful to mass the down going into each chamber. It's amazing how much down will compact to feel like nothing, even when you know there's plenty in there. I undoubtedly would have filled my down booties much more if I hadn't used a scale. (Which leads me to believe that using the scale probably would help minimize weight on future projects.) I will probably mass the down going into future projects; however, even down distribution and density are the ultimate guides to insulation effectiveness. It's commonly noted that chefs, for example, can pour salt into their hands within very close tolerances to the specified amount. I used the same sort of approach in this, and I think the end result works well. I could probably calculate the density of down in the vest but numbers make me shudder a bit. Given that the vest has about the same loft as my Summerlite but seemingly a much greater density of down, it would be really interesting to test the vest and see just how warm it is. I'll try to make myself sit down and calculate clo.