That’s a very good point. The problem with your average hip flask is that it’s really quite small, which means there is really not much space inside the flask to work with. The mechanism that springs to mind is the same as you find in a twist-up stick deoderant. Namely, you have a twistable knob on the base of the flask, which rotates a screw which runs up the centre of the flask. Mounted on the screw, inside the body of the flask, is a plate which, due to the geometry of the flask, can’t rotate, and will thus be forced to move up and down. Whilst ensuring the reliability of the seals may take some experimental work, this woild allow the user to retrieve the soup simply by twisting the knob, thereby compressing the soup, causing it to exit via the neck of the flask.