Yes. For me is like imagining having a semi-sphere of the size of your waist, inside you. You can play with that imagining diferent kind of liquids like oil (more difficult to spill with little vibrations), water (easyli spllled), etc.. to adjust it to your skill level. The point is trying to keep the liquid inside and the less vibrating you can.
Sure, but why stop at imagining? I think it would be worthwhile to actually carry the liquid. Otherwise, I can see a person with illusions about their "centeredness" (or whatever) imagining their imaginary liquid being perfectly still. A real liquid won't conform to your imagination; it will only answer the laws of physics.