As long as the extension does not then place the cleartext back into the DOM. I would also want support on my Android phone too. So I guess basically, it's never happening.
If I can't safely check my mail from random computers anyway, I'd just as soon remove the browser from the equation and use a GPG-compatible mail client.
I can imagine an extension for Chrome or Firefox that will not be updated without my knowledge and has been peer reviewed like gnupg.