[chias]

Is this materially different from requiring the user to have some random characters in the password, but for some reason making them type these characters into the username field where it'll be cached by the browser's autocomplete feature?

It seems like this is an amusing enough hack to do on non-sensitive sites, but I wouldn't do this on anything "real". When it comes to authentication, "hey I had this really neat idea" is almost always an immediate precursor to making things worse.

[MarkMc]

If the random characters are stored in the User ID field then 95% of the time the user just has to remember their password. It is only when the user switches to a new computer that they would need to type in the random characters. Wouldn't that be a significant benefit over having to type the random characters every time the user logs in?

I agree with your observation that "hey I had this really neat idea" is almost always an immediate precursor to making things worse. Almost.