To my knowledge.. no, Hoost never did any ground training..
However, I doubt that he never did any. I mean, he did seminars with Fedor at one point. I am sure he learned some stuff from all the MMA fighters and BJJ guys he would have been exposed to in all his time in the fight game.
I mean, unless he intentionally avoided it, of course.
And yeah Sundance, he would have been one hell of an MMA fighter I imagine. I guess it would depend on how well he could have adapted his whole game. Not every kickboxer can make a smooth transition like Cro Cop or Hunt, as you know.