I have already found a bug and may need some enhancements. I will be happy to do the work myself (and return the source code updates to you) with a bit of initial guidance. This is a very useful plugin!
Before you get too far with mplayerplug-in, please note that I have pretty much stopped development on it for at least 2 years. I have replaced it with gecko-mediaplayer + gnome-mplayer. Please consider looking at those before you make any changes to mplayerplug-in. gecko-mediaplayer also uses the newer npruntime api that is recommended by mozilla.
As for the socket, you should not be passing the id of the socket, you should be passing the XID of the window of the socket. From there the plugin takes over the X context.
You can find links to those projects here:
Thank you! After posting this message I looked around a bit and found a reference to the Gecko project. I will investigate it. Thank you very much for your prompt help!