OK, I did this- Created a new dir for the child theme of 2011, migrated the one thing I’d changed (the #access style) to a new style.css below an @import statement, cached a copy of my custom footer.php in the child dir, zipped up the original /twentyeleven, then uploaded a fresh copy of twentyeleven.
Then I switched the theme to twentyeleven. The menubar went back to black.
Tried to open the protected post- same thing, entering a password in the Password: box and hitting submit just seems to reload the page.
Nothing in the apache access or error logs related to this page, as far as I can tell.