Just getting back to this again...
Okay, that worked, sort of. However, it appears that the surface showing in that gable is actually the "interior surface" of the wall, rather than the exterior, I think because the base of the wall starts out as a virtually interior wall. From the inside of the structure the surface is also interior.
I'm attaching a screenshot so you can see what TFP things the wall segment is. Even though I extended the interior wall (blue), it created a new wall object (green).
Thanks again for the help!