Technically, you can enable it anywhere by using Redirector to prepend about:reader?url= to the URL, with a sort of white/blacklisting system using Regex. It may or may not be able to show a useful content summary, of course. There is a pretty accurate algorithm built into Firefox that evaluates if it does, and shows or hides the address bar button accordingly. Maybe an add-on can leech of the result of this algorithm?