There were already cc halfwalls before the latest patch/ep. Those were in fences. It should be easy to use those as a start and just extend the mesh up, toggle the option, and there you go. It's not something I personally have any interest in having (I play with walls down and only have walls up if I'm taking photos), but there's no reason it shouldn't be possible.
Of course, the worry you always have with it is if they finally add weather, any custom full walls used as externals would probably not register as keeping the inside weatherproof.
I've taken a look at how it was done in Sims 2 and it's actually just a script/resource file that defined a new wall ID/type. There was a package file that had a text list with the description you would see in the catalog. A txt file was placed in The Sims 2\TSData\Res\Catalog\Scripts\Scriptorium_Walls-Fences-Arches\
This is the text that was in the txt file for the "No-CutAway" walls.
# *Custom* No-CutAway walls
catalogTextResource 95_NoCutAwayWall 129
wall 95
defaultPattern "blank"
wallThickness standard
mayChangeSurface true
mayAttachObjects true
mayCutAway false
requiresFlatBottom true
requiresFlatTop true
submersible false
deleteTool partition
thumbnailResource 0 0x499db772 0xC9090520
thumbnailResource 1 0x499db772 0xe9261347
thumbnailResource 2 0x499db772 0xfeed5515
thumbnailResource 3 0x499db772 0xfeed5510
catalogTextIndices 0 1 2 3
If I understand this correctly, it actually wasn't a new "Wall/Object". It was simply creating a new wall id (#95) and then changing the properties on that id #. It is still a normal wall so would block weather, light etc. That all seems to be defined in the properties of the wall for Sims 3.
And even if it doesn't block weather in the future there really would be no reason to use it externally. As was mentioned, the cutaway view already shows external walls. Its the internal walls that I would like to see more of.