A more general solution would be to pass the RequestHandler instance as
a parameter to the dispatch function.  This would allow the function to
pick out more than just the client address.  To avoid breaking
pre-existing code, this should be made optional, perhaps by adding a
keyword to the register_function method.  Something like this:

  def __init__(...):
    self._include_request = set()
  def register_function(..., include_request=False):
    self._include_request.add(method)

Later, the dispatch function would be invoked like this:
  kwds = {}
  if method in self._include_request:
    kwds.update(request=self)
  [...]
  return self.instance._dispatch(method, params, **kwds)
  [...]
  return func(client_address, *params, **kwds)