Virtualization of a 'protected interface'

I have an abstract base class that provides public and protected pure
functions.

class MyBase
{protected:
   virtual void InternalService() = 0;

  public:
   virtual void PublicService() = 0;

   void CommonImplementation()
   { // Do something that depends on InternalService...
     InternalService();
   }
};

class MyImplementation : public MyBase
{protected:
   void InternalService();

  public:
   void PublicService();
};

class MyProxy : public MyBase
{private:
   MyBase& Backend;

  protected:
   void InternalService()
   { // Modify Backend.InternalService somehow...
     Backend.InternalService(); // <-- access denied
   }

  public:
   MyProxy(MyBase& backend) : Backend(backend) {}

   void PublicService()
   { // Modify Backend.PublicService somehow...
     Backend.PublicService();
   }
};

Unfortunately MyProxy cannot access the protected members of /another/
MyBase instance. Is there another way to do something like that, except
for making MyBase::InternalService public?

The idea behind that is, that references to MyBase objects (MyProxy) can
override some properties of the underlying instance and different
references may override different properties. The common implementation
part in MyBase contains a framework to deal with change notifications
and asynchronously requested informations. If MyProxy overrides a
property it must also intercept the methods to obtain that information.
But these Methods should only be called by the framework in MyBase and
by proxy classes.

Marcel