Question: Simple as that, can we emulate the "protected" visibility in Javascript somehow?

Answer: 
[Sure](http://webreflection.blogspot.com/2008/02/how-to-inject-protected-methods-in.html) you can. Here's another [example](http://blog.blanquera.com/2009/03/javascript-protected-methods-and.html).
What could that possibly mean? You don't have *classes*.

I suppose you could analyze `caller` to determine whether it meets some set of criteria for being permitted to call a method. This will be hideously inefficient and your criteria will always be spoofable.