Whoa, that seems like a shockingly good idea! It could look similar to what happens when you enter data into a password field on a non-https website. The browser can pop up a little box under the input when you start typing, which says something like "this is your first time visiting this website. Only enter passwords on websites you trust."
Are there any major downsides I'm missing? Signing up for accounts on new sites would certainly create false-positives, but if the warning is properly coded that doesn't seem like such a problem.