You could use the surface render with a fluid, although it is tricky. I think an easier way is to use blobby surface particles. Make the threshold on the particles high and use several large radius overlapping particles. The high threshold will keep the blobs from being too spherical. Create a separate hidden surface that is inset from the inside of the bottle by the width of the particles, and collide with this instead of the bottle.
You could do it with fluids, but this is simpler to control and set up.
Duncan