I think it could very easily be done with the right planning. What I was thinking was a 'Worlds of DungeonCraft' or something like that. Say there are 5 people in the Project. 1 Person would serve as the project co-cordinator and the central point of contact. Each of the other 4 'Developers' would each have their own 'world' at their disposal, each world accessed by the players via a Level 1 Dungeon called the 'Nexus' or something like that. It could be a room with like 4 portals in it. From there, each developer could be assigned a block of levels for their world (i.e. Dev 1 gets levels 2-20, Dev 2 gets 21-40, etc.) and each of the portals in the Nexus portals to the starting levels in each block. That could solve the problem of how PCs get around.
From there you could assign the same technique for Quests, special items, etc. That way each dev could do whatever they wanted, without risking overlap with the other worlds, and it would help to make the PCs transferrable between worlds without the need for transferring save files to different designs. That was a single save file can be used for all 'worlds'.
Now, to the problem of databases....I think the best was to solve that problem would be to have a central database which all worlds use, maintained by the co-ordinator. So if a dev needs to create a new item, spell or monster, he/she would mail the co-ordinator with the specs, any art/sound files and the co-ordinator will integrate them into the database and redistribute the database to all the devs. Am I making sense yet?
Anyway, like you said I think it can be done, it would just require some planning.
