[eric_bullington]

Fair enough, that's what I get for believing a Stackoverflow answer (even a highly upvoted one) without verifying for myself:

https://developer.mozilla.org/en-US/Add-ons/Overlay_Extensio...

So with Firefox, you could build the kind of add-on described by Mike.

But I have confirmed for myself Chrome extension API's lack of ability to even read the certificate of a current page[1]. Chrome may be able to read block page loads (don't know, haven't checked) but without being able to even view a cert, it doesn't do much good.

1. https://code.google.com/p/chromium/issues/detail?id=93636