Dust multiplier only works on galaxies that are created from a galaxy spawn. Like you said, loading a saved simulation with various dust multipliers has no effect on the galaxy dust count.
I see the problem, however there's no easy way to fix this at the moment.
Check out the xml file in the galaxy collision ubox file. These galaxies are created with 'spawns' and are effected by dust. You could setup a simulation to test. Then modify that save (using the positions and velocities of the galaxy object) and convert them to 'spawn' galaxies.