In my experience Abide, if
In my experience Abide, if you can run 15 barefoot with no problem, you can probably run 26.2. I would carry some huaraches or other "rescue shoe" just in case, because after a lot of high mileage you never know what kind of terrain will mess up your feet.
I would try to get in another long run pretty quick here the 14-16 mile range, and then do as much of a taper as you can the rest of the time.
Who knows if that will work, but it's about all you can do.
Oh, and Freeyourtoes ran the marathon that he did with me. He only ran around 12 miles barefoot, and the rest in huaraches. But he didn't have nearly as much of a barefoot base as you did. Longest barefoot run was around 4 miles or so I think.