I asked this question last year, but cannot find the thread. I managed to get this to work then too! I even have the model still but I cannot do it again on another model.
I'm simply trying to get a cable railing to display correctly where the newels are on the outside of the deck edge attached to the rim. This was done by drawing a deck first with an "invisible" deck railing, then draw another rail wall outside the first, and snug it up next to the invisible railing.... BUT.... under the structure tab dialog box the default wall heights dbx is grey out on the one that will not extend below, whereas the one that does extend below the top of the deck, the "default wall bottom height" is unchecked. I don't recall why or how to get that to work.
Any one know???