Tom Ritter:
It seems reasonable but my first question is the UI. Do you have a proposal? The password field UI works, in my opinion, because it shows up when the password field is focused on. Assuming one uses the mouse to click on it (and doesn't tab to it from the username) - they see it.
Depending on how "intrusive" you want to be you could hide the bitcoin/onion addresses with an overlay similar how NoScript (used to?) hide flash content and make it visible after clicking on it and acknowledging a warning.