As someone who has organized and done the dishwashing for gathering of 50+ people in very basic settings (outdoors, using plastic tubs and communal washing faci

As someone who has organized and done the dishwashing for gathering of 50+ people in very basic settings (outdoors, using plastic tubs and communal washing facilities) I can tell you that the grease under the plates are largely irrelevant. It makes no difference whether the grease is stuck to the upper or the lower side of a plate. Much more inefficiency is created by people not collecting plates, not sorting them but building arbitrary piles with everything squeezed in between plates, and dropping their plates and cutlery into the wrong pool of water. Because what you want to do is, where possible, make it so that everything goes through several stages, the first being a tub where all you do is wash the grossest stuff off; this can also be the tub where you tell people to drop everything into (except the napkins). Space allowing it is actually advantageous to not drop cups and glasses there as they tend to be much less greasy and yes, that can make a difference. So the challenge is in fact partly to keep surfaces from becoming greasy, but the bottoms of plates are not a a problem. If you pick up the plates from the tables you have to stack them anyway, because it will not be very practical to have 50 plates on a table right next to each other. Just try to treat cups and glasses separately and you'll be good.