The "other method", which you mention, would not be easy. And I'm not at all thinking about the people on the ground.
But releasing that sort of stuff from a pressurized cabin, and into the temperature at cruising level, that would certainly be a technically very demanding job.
And over anything but a desert summer the frozen "stones" would be potential killers when hitting the ground.
In fact there has been incidents when stones from leaking lavatory service valves hit the ground.
At lot of things have changed since the old days when wing walkers discovered that sitting on the trailing edge was better - or at at least more economical when talking about soap for shirt washing.
But glider planes actually often have a tube to the outside for the light jobs only. When flying really high (in mountain waves), then the tube may freeze, and your only choise is to land in your "bath tub". And until then pray for not too much turbulence and negative G force.
Cheers, Preben Norholm