Well, apparently with Home Designer Suite 2017, there is no ability to attach a molding to a soffit. I also couldn't make sense out of other posts recommending other methods like "No Room Defined" or "Creating Virtual Rooms". As a last ditch effort I tried various combinations of random stuff and came up with something that might just work. I say "might" work, because although I can now trick the software to not display a chair rail on a specific wall or walls, I have no idea what kind of problems this could lead to later in the project!
Anyway, here is my potential solution:
(You should already have a room created)
Start by creating a straight interior wall, in front of the existing room wall you don't want the chair railing on
Edit the straight interior wall you just created (not the actual room wall) and change the following general values:
Thickness = .125"
Select the checkbox for invisible
Wall type = Room Divider-1
View your room and the chair rail should be gone from that one wall. For some reason though, only the chair railing is gone, which in this instance is what I'm looking for, but sometimes that may not always be the case. I added a crown molding to the room walls just see what would happen and the invisible wall also blocked it from displaying on the wall. Strangely, the base molding does not get blocked when using my invisible wall trick. Possibly, the sequence in which you choose these options may change the behavior? I have no idea and to test all the possibilities would be more work than I have time for right now.
I really hope someone has a better work around than what I've come up with, but if there is one, I can't find it in this forum or anywhere else on the WWW. Better yet, ChiefArchitect should add this feature! At the VERY least, when you break a wall, the broken section of the wall should respect changes to the moldings, just as it does for color and textures, etc . . .