what I do is add a Mass Element with its correspondent geometry to the face of the walls that need it. It easier than creating a new component to a new wall style. This has the advantage of setting the proper material to the wainscot commonly different from the wall material. And, besides, you gain control of its area apart from the wall itself at the schedulling time.
Just a point of view, could be others.
I am assuming you are doing this for 3d only, so I would add as a comp to the wall... You can still change the material in MAX and you wouldnt have to model it then add. But the other way is easy too... Take your pick... :-)
the easiest way I have found to do stuff like this is to create a wall style that is as thick as the wainscoting. Then for the top and bottom add a sweep for the trim to it. It works GREAT! its quick, easy, and can run anywhere and everywhere you need it.
I agree with the wall style idea. It cleans up with itself, you can mitre it easily when attaching a sweep.
You can attach as a componant in the exisitng wall style but I found it a lttle tricky if you want to have separate control from the wall. Just add a new one with a different cleanup group.