I don't know why you would want to walk a spear into someone rather than thrusting it into them. That doesn't make a lot of sense. The whole idea of couching on horseback is you are bracing the spear and using the horse's energy to deliver the blow. But you can't generate that kind of energy on foot so you add more by thrusting the spear into the opponent as you charge him and that's reflected in the speed
What pro-couching on foot people are really talking is bracing the spear for impact on foot. Yes, walking it into someone isn't going to do much. What you are trying to accomplish is to get that huge mounted enemy to use all their momentum to skewer themselves into your long pointy stick.
That is why I would restrict it to slow movement at the most (if not lock them in place with only the ability to turn). If the engine allows it, I'd let braced hits do no damage to very slow speeds (such as standard movement on foot).
The point is not to walk/run your "couched" (ie braced) spear into your enemy. It is to use the momentum of the enemy against themselves.
Charging into a wall of pikes should be a very deadly thing for mounted soldiers.
So really we are talking about bracing, but mechanically it is pretty much couched lance in this game.