Should the ServiceContainer allow the app to overload default extension classes?

[g105b]

Specific example: My app deals with editing HTTP Requests, so I have a class called EditableRequest that extends Gt\Http\Request. Because of this, the EditableRequest class is instantiated and passed to the DefaultServiceLoader as the actual Request, rather than the actual request that's coming in.

This is obviously a flaw in the system, because it should be fine for the App's service loader to supply classes that extend already-existing classes.

The question is, what should the default behaviour be? If there's already a class defined in the default service loader, should we ignore loading any extras that simply extend the base classes? I think that would be safest.