I've tried that thru-hiker pattern before. The main problem with it is that it is made for fabric that stretches, and while we know sil does stretch, it is the wrong kind of stretch. So when you use them, the thumb doesn't articulate well enough.
Lance, that pattern looks great though. I've been looking for something that had a thumb that was separate from the rest of the mitt like that for a while, and your pattern looks perfect. Thanks for sharing it with us.