So after answering this question "Bodyweight exercise for the 'v'-muscle in the lower abdomen" I started doing some investigation into the transversus abdominis (TA).
From what I've found, while the TA provides support during any heavy weight bearing activity (squats, deadlifts, overhead press, carrying shopping), the only way to directly exercise this muscle is through abdominal vacuums. However there is really no way to make these harder as the TA acts as a girdle around the abdomen, with no way to add weight or change leverage. Although some people think doing this on all floors so the body sags more increases the difficulty, but not by much.
So the only way to really train it then is through volume. Starting a few days ago I started doing vacuums pretty much constantly, sitting at my desk if my posture lags, waiting for the bus, while cooking, etc.... Since then, I've noticed my posture is a little improved and my stomach certainly feels tighter (but that could be DOMS) and I think I look slimmer.
The question is, what physiological changes happen from exercising such a small, important muscle in this way, are there any dangers or benefits, will it interfere with other lifts?