Proxying una clase de Java con una conversión de clase interna

Here's a problem I am facing.

I am writing a plugin. There is an interface called SystemObject, and a default getter.

public class MyPlugin extends Plugin {
        @override
        public SystemObject getSystemObject() {
                return super.getSystemObject();
        } 
}

SystemObject interface has a method called getScreenSize() which I would like to proxy or intercept. When I create a proxy class, or simply implement this SystemObject interface myself, I get a class cast exception.

This is because the caller for getSystemObject (part of the plugin system) has this in their code (found via reverse-engineering):

private void foo() { 
     SystemObjectImpl impl = (SystemObjectImpl)plugin.getSystemObject(); 
}

Mi pregunta es: is there any way I can proxy calls on the SystemObject interface?

I tried implementing the interface and using java reflection proxy invocation to no avail. Unfortunately, I'm not responsible for running the java process, so I can't use an agent.

¡Aclamaciones!