If you're going to the trouble of subclassing SdistBuilder, it's probably simpler to use that interface directly, which saves monkeypatching it. It uses pathlib objects.
sdist = SneakySdistBuilder(Path('pyproject.toml')).build(Path(td)) tarfile.open(str(sdist)) I was pointing to the PEP 517 interface because that's more likely to stay consistent, but if you need to monkeypatch to use it, you're already relying on the underlying APIs.
Other than the PEP 517 interface (exposed as flit.buildapi), I don't consider any of flit's Python API to be public. I don't expect what you're doing to break any time soon, but be aware that you may need to update the script as flit evolves. Hopefully tox will add support for pyproject.toml, so you won't need it for long.
Finally, you may need to regenerate the setup script when the version number changes, or if you add subpackages, or folders of package data. And possibly some other conditions that haven't occurred to me. It's drawing on more information than just flit.ini.
We don't need to change anything now, and maybe ever, but I wanted to capture this comment just in case. I'm hoping tox will just consume pyproject.toml soon enough and we'll never have to worry about it.