Injecting Local Variable To A Function

I'm looking for a way to define a function that's only effective inside
specified function.

Featurewise, here's what I want to do:

bar_plugin_func = function() { ...; setTimeout(...); ... };
wrap_func(bar_plugin_func);
bar_plugin_func(); // calls custom "setTimeout"

So while plugin developer thinks s/he's calling top-level function,
I want to hook it by somehow injecting local variable with the same
name.

Simply doing something like

;(function() { setTimeout = function() { ...my custom implementation
})();

is not ideal, as that'll taint global scope. Actually, I can live with
that for now,
but it's my technical interest to find a way to define it locally.