I'm using the new pressure settings for cloth sim in Blender 2.9 and I'm able to use a moving object with collision on it to squeeze a spherical shape with cloth and internal pressure applied to it. It's fairly convincing except that if you view the deformation from the side, it squeezes realistically at the point of pressure, but the overall volume is decreasing. On a real balloon the sides would inflate outwards around the point of pressure. Similar to the way this toy inflates in areas that aren't being restricted by pressure: https://www.youtube.com/watch?v=fA7_78PouHw
I'm trying to do a visual demonstration of this effect for a presentation, is there a way to tweak the settings to achieve this using physics, or barring that, a way that I could convincingly fake it? My best guess at the latter option would be to bake the physics animation, then try to add volume to the sides at the point of it being squeezed in the animation. But I'm not sure if this approach would be worth pursuing, would it be possible to do it with shape keys and animate that?