The branch feature/resizable-terrain on my repo implements this feature, including with nine-slicing support. In the process of writing this, I also slightly tidied up the resizability code for objects and the general orientation code for terrain; these improvements have been ported back to the master branch.
I have not yet implemented editor support, for two reasons:
1. The editor master branch doesn't have nine-slicing support yet
2. Passing everything to the constructors in the editor code is starting to get really messy; I'm wondering if this way of working should be revised before any further new features are implemented in it...
EDIT: If I remember correctly, I had to delete this because it became too incompatible with other changes - but shouldn't be too tricky to re-implement again at some point.