I understand the need to accommodate a head that is larger than your neck. But, looking at how the foam pad splays out during the night, what if you don't cut the sternum-to-belly-button cut in the foam? But instead cut some "fingers" around the neck so that (1), they flare out of the way when you don it or take it off but (2) provide insulation as a vest and padding as a sleep pad.
As far as Justin's concern about sweating: I'd imagine that the outer windbreaker is a huge factor in retaining or shedding heat. Worn just as a vest, it would add some warmth, but only with the windbreaker does the assembly become a jacket.
My joke about a PFD was somewhat serious. It doesn't come up much for most BPers, but in a paddle raft, canoe, or potentially on a charter flight to a fly-in trip, the safety advantage of having, essentially, a personal-floatation device on at all times, is HUGE. Flying with the USFS in SE Alaska, it was required to wear an inflatable life jacket in any plane without two engines, two pilots, flying IFR. i.e. you wore a PFD for every trip in Alaska until you got back on a 737.
Edited to add: I'm having images of cops wearing "bullet-proof" vests. Because those are somewhat bulky and planar but have great velcro closures to seal them together. It makes me wonder what the thinnest, lightest velcro-like material is and could you use it to seal up the side so (1) you'd be even warmer and (2) it would have more warmth as a vest.
I've been tempted to try one of these for a year now. This might motivate me to try my own version. Thanks, Daryl, for the great post.