most walkers are ridden saddleseat because of their gait. I've seen some (ok, a lot) of pleasure horses ridden western, too.
I think the main reason you don't see a bunch of walkers ridden huntset is because they don't trot. (so there is no need for posting). I don't see any reason why you can't put huntseat tack on a walking horse, but "theoretically", the walking horse should be under tack as to exhibit it's smooth gaits.
Does that make sense?