But what if you'll make them use two more tiles like "Only vertical" and "Only horizontal"? And use them like "inner_corners" by '.frames: 4' (normal crumble with outer corners, crumbling inner corners, and then only horizontal and only vertical in any order or not). I would also perfectly affect to [border_size <= half of tile_size].
I think it would be very nice and almost perfect feature for professional RND-graphics makers!

But sorry that I told you this pretty badly and didn't even show any examples, I just wrote it on my phone...