I don't know if this pertinent or not, but I used to like to lay on my stomach and breathe "into" my kidneys and lower back. I can see how this probably wouldn't be quite the same thing as being able to control the expansion of the abdomen directly, but it was an interesting way of seeing where the expansion can go. I use this as a way to help me put ki into those areas so I can be a little more aware of them in general, so for me right now it's a way of mapping the body.
p.s. the first time I threw my back out was sitting down; it's the same place I usually feel any lower back pain today. All it takes is for one critical piece to get strained or otherwise get out of whack for the house of cards to start falling down.
Stuart McGill knows his way around backs and core stabilisation better than just about anyone. disclaimer - I don't get any royalties from his book sales, I do coach at the rowing club where his wife and daughter row.