My gelding, Mingus, has tender soles. I plan on getting boots eventually, but we've taken a couple of preliminary steps.
First, we now leave his hooves a bit longer. Not much, but even a little bit helps.
Second, when we put down gravel as mud prevention, we included the area surrounding their water troughs with the idea of toughening up his soles. As it turns out, our horses LOVE the gravel, so we'll soon see if it helps with his tender feet.
I know the Build-A-Bear shoes work for nursing home visits and the like...do they really hold up to driving, especially over gravel? I would be afraid they'd break down or not provide enough support.