M444,
To test like a "plug-in", your program would have to be a "plug-in" for the browser. Testing for anything beyond the browser would be to access a security hole that has long been plugged. If you're program doesn't interact with a browser or an application inititated through the browser, then you're out of luck.
If it is a true "plug-in", I would think that you would have to have the "plug-in" source code for the browser, or some other SDK type utility to build the plug-in as well as provide a method to download the plug-in.
There's always a better way. The fun is trying to find it!