I totally agree about the splitting and that there are many other woods that are "preferable for a custom stock". And you are very correct about shaving the wood away to a single growth ring when making a self bow. I, like you, have seen fence posts that have been in the ground for years and are still solid. This was the post of choice when I was growing up, building fence since I was the labor for cutting the post and digging the holes and trees are prevalent where I was raised.
But with all of that said, I still think it would make an interesting stock if I could guarantee a process to maintain that fresh-cut, yellow color. Call me crazy, but I'm nostalgic when it comes to custom gun calibers, shying away from the latest and greatest craze. And yes, there is nothing more beautiful a well made walnut custom stock, but I like the obscure; and I think a stock of Bois d'Arc meets that criteria.