I *think* Durasole, which is a sole hardener, has formaldehyde in it.....among other things like iodine and genetian sp? Violet.
So I could conceive, in a home-remedy sort of way, of someone "injecting" the outer portion of the hoof (like the frog crevices and such) with something along those lines. Not that it would be a good idea.....I really have no idea. But I could see where the thinking comes from.
I actually have wondered myself, if the long-term use of Durasole is okay for the horse......to use in a thrush-preventative sort of way? I tried it out as a sole-hardener, and it definitely works, but I don't need it in that fashion anymore. But, it works wonders for opening up the central crevice of the frog and helping it dry out. My mare actually has really healthy, normal looking frogs now (before I could never get rid of the central crevice). So I really love it as a thrush killer.....it works much better than Koppertox. It actually dries out the problem area and lets it get air.
So anyway, I have heard of formaldehyde (formalyne ?) being used on hooves. Can't comment on how safe it is for long term use, I have been wondering that myself. But I believe it is an ingredient in sole hardeners.