You can also do it with the method I thought of earlier, where you basically put the whole construction script in an editor callable event. It turns out, you can also use blueprint interfaces during construction, so you can keep everything independent.
I got as far as remove instance, but ran out of steam before I could get it to regenerate the missing boxes ( the 3 cubes are tiles 
and the collision box is stairs )
So the construction script in both is
The BPI is
The ‘tiles’ BP
and the ‘stairs’
In a word, nightmare. Which is why Code Respawn did it as a plugin