Soft Body Alone vs Soft Body Holding Fluid or Semi-Fluid

I’ve been looking over some of the threads about soft-bodying vertices on a character to simulate fleshy jiggles. If my understanding is correct, the soft-body physics is one of regarding a mesh as a hollow truss, then setting for various progressive resistances to lengthening, shortening, or angular motion of each truss member with respect to its neighbors. To keep it from collapsing one needs to conceptually add springs on the truss joints which restrain the collapse. But the real situation is more like a thin flexible vessel holding a fluid or semi-fluid, and it is the conservation, or effective compressive strength, of this fluid or semifluid which balances a compression in one direction by shearing until the shear stretches the vessel transversely and the vessel’s tensile force sufficiently pressurizes the fluid, in the vein of the ellipsoid water balloon at rest on a surface under gravity.

My question is, is it possible to, in so many words, model a fluid poured into a soft-bodied mesh to make it such a vessel and have the two press on each other? Is that making things too complicated, or does it have the potential to act more accurately than a soft body mesh all by its empty self?

Bag like this?

http://www.antigravitygear.com/images/PRHY-waterbag1300.jpg

Are you trying to simulate fluid filling the bag?