I have been doing a huge amount of searching through all mods, file edits, model importing, model databases, anything have to do with npcs, a lot of stuff the last 2 weeks. And I have found a lot of things I was not looking for but were interesting, I must have 150 bookmarks.
But this is the first mention or hint of any type of mod that lets you build walls on unflattened tiles.
You can build on flattened slopes of course. Otherwise it might be something a server team made for their own server - and that type of thing they don't share - so you might want to find out where/who even told you it existed and start tracking it down there.
Or a link?