I think this would be a cool feature, but I can't do server side stuff ): . Often people will make group accounts and give all the members the password (through the conversation feature on the website). This works fairly well, I know of at least a few saves that have been done like this (I think there's some electronics club that does this). It tends to be one person doing most of the work still but other people help.

Also for some self advertising, you could try TPTMP (TPT Multiplayer). Just have everyone go to a room when they want to work on the group save, if they are the only one there they can work on it alone until someone else joins. To get TPTMP just download the script manager and use this to download the TptMulti script (first one). Anyway, once you connect you go into the lobby, but you can type /join xxxx to join a private room xxxx. You can also use /msg to tell other people on tptmp what the room name is.