Have you tried making a cylinder roughly the size of a bale with a density such that it would weigh about the same? Then 'clamp' it with a force to match the friction between bale and arm. Apply gravity to the whole thing and fix the attach point(s). You should probably take out the cylinders, apply appropriate forces on the mount points and add a remote load with the cylinder weight.
What you could do is fix the area of the jaws that would be clamped round the bale at the correct distance apart. That way you can actually get a reaction force from the fixed area.