I have to admit, I've never kegged before so I'm making an educated guess here.
The reason a diffusion stone works aerating your wort is because of the velocity of the oxygen running through it. If you were to turn on your shower all the way you get a separated spray. But if you turn it on just a little you still get a single, collected stream, defeating the purpose of the shower head. I would think a diffusion stone in a keg would help a little when you first apply pressure and the CO2 is steadily running through the stone, but once the pressure is to a regulated point, it's only very slowly running through that stone. It would seem it wouldn't make much difference at that point.
The reason a diffusion stone works aerating your wort is because of the velocity of the oxygen running through it. If you were to turn on your shower all the way you get a separated spray. But if you turn it on just a little you still get a single, collected stream, defeating the purpose of the shower head. I would think a diffusion stone in a keg would help a little when you first apply pressure and the CO2 is steadily running through the stone, but once the pressure is to a regulated point, it's only very slowly running through that stone. It would seem it wouldn't make much difference at that point.