It seems that VTK developers would not like to introduce yet another scene file format (and I fully agree that it is a much better investment of time to improve support of existing standard scene file formats).
This means that @amaclean you need to resist the temptation to add more feature to this new file format (to avoid making users depend on it) and make sure users understand that it is just to be used inside some VTK tests. You could add a warning in the file format description and direct people to use standard scene file formats instead (or maybe remove the file format description page from the public documentation to make it clear that it is not some officially supported VTK file format).