PyPI mirror proxy that injects code and bypasses pip hash verification

Yes, if you control the index, you can lie to pip about what the package's hash should be. This is why you have to opt in to using a different index, and why the connection to PyPI has been properly secured since forever (https://github.com/pypa/pip/issues/425 ; note the date).

Once pip supports installation from a PEP 751 lockfile (should be very soon, by my understanding), presumably this won't work, unless the lockfile is already compromised.

The clearly AI-generated README is also confused about how this works. It claims:

> Intercepts package index requests and rewrites URLs to point to the malicious mirror

but it's actually implementing a malicious mirror by forwarding requests to PyPI and then serving a modified version of the PyPI result. "Preserves and updates SHA256 hashes for modified packages" is also an incoherent description; preserving something and modifying it are mutually incompatible.