If we’re going this way, I’d vote for tweaking it slightly to make python-path a list, like $PYTHONPATH is. If we think a common pattern is for a selfhost.py that does nothing except add extra entries to sys.path and then redirecting to the real backend, then we might as well let you write both paths directly in pyproject.toml:
[build-system]
requires = []
python-path = [".", "vendored"]
build-backend = "setuptools.build_meta"
If projects want to use the selfhost.py version because they think it’s clearer, then they’d still have that option.