Read my next paragraph. A split of public and private data is only marginally more difficult, and is very helpful:

    CREATE TABLE public_user_content (
      body VARCHAR(512) NOT NULL,
      user_id BIGINT REFERENCES users(id)
    )

    CREATE TABLE users (
      id BIGINT PRIMARY KEY,
      username VARCHAR(128) NOT NULL,
      email VARCHAR(256) NOT NULL,
      badly_hashed_password VARCHAR(8),
      social_security_number VARCHAR(9)
    )

    GRANT SELECT ON public_user_content TO public_readers;
    GRANT SELECT ON users (username) TO public_readers;

Then in your view:

    @run_with_role('public_readers')
    def get_public_feed(request):
        ....

SQL injection in `get_public_feed` is no longer part of the attack surface for stealing PII.

This is a good idea.

However, in the context of forms authentication, there comes a delicate moment where the user name and hashed password must be exposed to an unauthenticated user, no?

Of course. And once one user logs in, if you have SQLi vulnerabilities in the `edit_user_pii` page, the attacker has read/write access to everything. Like I said, it's a way to reduce your attack surface, not a way to completely lock everything down.