I personally haven't heard of it and think it would be detrimental to a muay thai practitioner because it can only develop power on a vertical plain of alignment, and also if your going to condition your hands the traditional way (the first 2 knuckles) then this is also not good for muay thai because it's a very active and often unpredictable sport, you can't predict what knuckles you are going to hit with in a fight.
Also with a makiwara you have to hit it flat on, I personally found this factor to be hinderous when I hit something more representative of the human body (a punch bag) because where I was used to hitting things flat on, the knuckle of my thumb would actually hit the bag along with the knuckles and be pushed back and it would either get cut or hurt where it was about to break (never did though).
With punching you want to develop 'power lines' from all different angles, not just the ones that the makiwara will allow you to.
The way of the warrior is a resolute acceptance of death. -Musashi