Thanks Gdog42. I brought my monstrous board into AnyMountain last night for tuning (because I'm too lazy to do it myself). While there I found a pair of size 15 Burton Moto boots which I tried out in my adjusted NX2-GT XL binding. Being newer and having that footprint reduction just wasn't enough. However, it did come closer to fitting than my 2005 DC Judges. I'm just not sure what the deal is. Either I'm doing something wrong, my pair shipped with the wrong sized toe straps, or as I strongly suspect Flow is guilty of gross false advertising. I've emailed Flow with what I found but they've been unresponsive.
Wiredsport, I used one of those shoe store type foot measuring plates and it shows me at just over size 14.
Another great thing about Burton is their bindings. I saw a size L pair in a shop last year was was considering them, so I called Burton. They said that although the bindings are already pretty big (true), if they're not big enough they'd send me a set of even longer strap connectors and ladders for free.
Those guys are awesome! So maybe that might be better for you?
In the end I got a pair of Union bindings. With my size 14 boots, they fit with even toe and heal overhang, with the heel loop and toes ramp extended fully (the heel loops came like that). The toe straps are on the second-to-last adjustment holes, and so are the ankle straps. So there's still likely to be room for a size 15, depending on how much larger DC boots are compared to Ride. I think Ride boots are a bit larger than others of the same size.
Check out Burton and Union bindings if you can!