How about, in the same way that some banks allow you to upload an image that they can then show you to prove you're talking to the same party that you've talked

How about, in the same way that some banks allow you to upload an image that they can then show you to prove you're talking to the same party that you've talked to previously, a chromeless browser could ask you to "draw" or supply an image for trusted actions. This "badge of trust" could then be show inside any dialog that asks the user to perform any action that you would not want a website to fake. It would of course be important that untrusted website content is never able to read these pixels. This is essentially inverting the relationship, by inserting a "zone of life" inside the "zone of death".