I was looking through wall pieces and found one that wasn't tied to a tileset: it's the unburnt version of 1m_Stonewall_Burned_A:
It looked like something a lot of people could use so I thought I would share how I got it working:
1. Find a tileset (wall construction) in the root templates and "Create new from selected"
2. For "Tile Set", click the button on the right to show the "Choose TileSetPieces" window
3. Find all the Stonewall pieces in the resource manager and add them to this window. They are all Visual Resources with names starting with "1m_Stonewall_A" and ending with "Wall_(number)_(letter)"
If you've done this correctly, you should have a new tileset.
Alternatively you can put this file
into Data\Public\ProjectName\RootTemplates. I think that should work.