Ok, here are my thoughts.
First yes just adding weight will help with free recoil as well as felt recoil. Mercury recoil reducers rely on the slosh of the mercury to effect a reduction of felt recoil. So if you have a pipe with shot in it that should (in theory) provide the most recoil reduction.
Now in practice the fixed weight of shot tightly contained can be used to balance the rifle to your liking, while reducing recoil. Loose shot is also noisy and I think it detracts from the gun. (there are just a few noises a gun should make, click, and boom are the typically acceptable noises, while a rattle is not acceptable)