func_get_arg() returns a *copy* of the argument, to my knowledge there is no way to retrieve references to a variable number of arguments.
I have a module system in my game at http://lotgd.net where I'd like to be able to pass a variable number of arguments to functions in a module, and pass them by reference if the module asks for it by reference, but you can't accept optional parameters as references, nor can you retrieve the reference on a variable number of arguments. Looks like my modules will have to do with out the ability to accept parameters to their functions by reference.