I've envisioned this a while back for if I have a stock that I want to make maximum use of its length. It can be used conventionally by someone who doesn't have a problem with chest loading putting a butt directly on the end, and can be quickly converted to a hip loader by putting on the pipe butt extension. I made a little photoshop of what it could look like.
Recently I chanced on this pic. It's more complicated than what I had in mind but looks like others have been thinking in the same direction.