Yes. It is actually very easy to extend Smalltalk to support private/protected methods (message bytecode based on the protocols, for example). But Smalltalk would need to do these checks in run-time because all message sends are late-bound.

Thanks! Can you point me to where I can find out more about the eval-"loop" that the entire Smalltalk runs on?

Check the Blue Book: http://stephane.ducasse.free.fr/FreeBooks/BlueBook/Bluebook....