Judging by the lack of replies, I don't think it is commonly done. Of course, with enough time, effort and money, anything can be done.
I think you would pretty much have to do a complete re-build of much of your Airstream to do this.
Here are some pictures from someone doing a complete restoration. You can see the frame structure under the floor.
Airstream Forums - View Single Post - Doin' the Full Monte
I think I would buy one that has the bathroom in the right place before I put that much time and money into it.