The npapi plugin is a slight modification from FBTestPlugin.
The plugin is currently defined public in manifest. Now, the plugin method can be called from external page. But not in any page in the extension, I've tried background page & option page. There is a method runProxy() in option page, it does the same thing as the external page, but no method can be found by plugin object.
You may download the chrome extension here, and try it out.
I debugged in Linux, with command line
sudo /opt/google/chrome/chrome --plugin-launcher='xterm -e gdb --args' --user-data-dir='/tmp/liruqi'
And found this maybe the problem.
[6958:6958:10328544637:ERROR:extension_prefs.cc(955)] Bad or missing pref 'state' for extension 'mpeobcmgemcjlkjkmgialddejohkigkg'
But, I wish the plugin can be defined private(only called extension page) for safety reason. how to fix?