The ZTS
I may be able to bring back the nude open tops and fix all the clipping issues with all the outfits that you choose to stay non nude.
You can change the item's individual childgeo to use the F basic chest or hips. For example, right now if you make the sierra open nude, only the neck and shoulders will render because it uses F chest v neck arms.
Password42 got around it by modifying F chest v neck into F chest basic, but that causes clipping with any other open tops you leave non nude.
At first I though that changing individual childgeos would cause an in game error "invalid outfit type" but it seems to be working. I am working to redo the entire Costumegeometryclient. The belly geos and skirt fixes will no longer be needed if this turns out as expected and we could make virtually every outfit nude with no clipping to others. For those who want nude NPCs it might be very well possible.