How to overload the assignment operator?

Thank you Brian,

setReplaceMethod() is just syntactic sugar for setting an S4 method on a
replacement function (read the function definition to see so).  You can
set S3 replacement methods, and the S4 mechanism just piggy-backs on that.

So I understand that setReplaceMethod will not help.

I think the conceptual problem is that the assignment operator does not
actually do any copying: it creates a binding of a symbol to a value.
Any copying which occurs happens when the value is (potentially) changed.

I would be even happier if the cloning would only occur on any attempt to change the external pointer / proxy-object.

There is no provision for that to depend on the class (rather than the
type) of the object.

Mh, unless the internal copying mechanism would call a clone generic for non-atomic objects

Since external pointers are never duplicated, you
ought to be able to take advantage of that to design the copying semantics
you want.

How that? How do I know that any user has assigned/modified the external pointer (or a proxy object containing the external pointer) such that I can invoke the cloning?