From what I have learned here and some other places, yes adding weight to the rear of an Airstream is a "no no"
That said, with a little work you can do what your asking about. You will need to re-enforce the frame from the axles back. This will distribute the extra weight along a larger section of the trailer. It is not easy, but can be done.
1967/8 Overlander International Twin w/ bunk/s.
Yes, four kids and two adults in the thing.
Happy wife, happy life.