Machine Oil will. I've used it before... the 3-in-1 Machine Oil. It's very strong smelling and very greasy, but doesn't harm the horse at all and I've had good results with it... not as good as with MTG, but still pretty good. It also helps make the mane/tail thicker.
I don't know what MTG is, but gelatin will help and almost any hoof-grow supplement. Anything that you apply to the hair, rather than feed, may help protect the existing hair from breakage, but won't encourage growth.
Someone posted about some stuff called Nu-Stock I was thinking of looking into, but it is very similar to MTG. I also want to try EQyss mega-tek rebuilder if I can find it at a decent price. I have one filly whose mane and tail just wont grow past a certain point before breaking. http://www.eqyss.com/mega_tek2.asp
MTG is a topical ointment that contains sulfer, emoliants and skin conditioners. I think it just makes the skin more conducive to growing hair rather than actually some magic bullet that grows hair. If it did I'm sure humans would be using it for baldness