How about this. When the user creates a class that implements the interface, they must also register themselves with the Event Controller. This would let the Event Controller know what specific objects it should be calling. This would also be doable without forcing the user to modify the Event Controller class themselves, which is what you want I believe.

You will not be writing any files (which you cannot do on the site); you are just reading one that is within the project directory (which is allowed).