How do I check in Firefox and Chrome if a plugin is installed
Have you had a look at window.navigator.plugins?
..and the equivalent reference for webkit plugins-
The Apple reference for the WebKit DOM appears to have disappeared. navigator.plugins does work for me in Safari & Chrome. See a jsfiddle here: http://jsfiddle.net/2EaKD/
Sounds to me like a potential security hole, if a website (something remotely) could access your plug-ins (something locally). So I guess this is not possible.
Your addon can monitor which pages are opened by the user. If it is your page, you can make your addon set a global variable in that page and your page's JavaScript should be able to detect this.