However, git does not provide an easy way to pull a subfolder, and there's some trickery required. We decided to delay the implementation to keep our build system simple and more maintainable, and would probably use subtrees or submodules as an alternative.
Sorry to have missed your original ticket.
Looks like you found the clone_script override option.
I agree that submodules would be the simplest way to go if you want to break a large repo into smaller more manageable projects.