Personally, I think it comes down to genetics. I've tried all sorts of products when I would show.
You can put whatever you want on there, topical solutions DON'T make the hair/hooves grow any faster. It can improve the condtion of the hair shaft, sort of protect it from breakage and make it seem like it's growing because it's not snapped off, but, we are talking about dead cells. The real factors are gentics and nutrition.
If it's greasy enough, it will weight it down to make it appear longer, but you are wanting the top of the tail to grow, and it just won't happen. If she's rubbing her hiney on something to break off the hairs, investigate and eliminate the cause. Otherwise, I think you just have to live with it.
And I've tried MTG. It's an okay product that has its uses, but it stinks all the time and I've not seen the "miracle growth" that it boasts.
I think you're just as well off, if it's easier, to use human conditioner, and keep her washed and clean.