What is Metatrader plugin? I have seen the term in several promotions and articles. What was described in these articles was a species expert with inserts of libraries in other languages (not MQL4 / 5).
A plugin is usually referred to as a c++ extension that runs in the MT server it self.
Just like the meta manager api, that allows you to extend the operations of the platform as a manager, an MT plugin allows you to extent the actual trading logic of the server. Of course it goes without saying you would need access to the actual server to deploy one.