I disagree. Instead of cheating, lets just make it a group project to mine out a huge ass cave. We can make a large underground base camp with a huge vault to put all of the cobblestone into, and maybe an underground tree farm to supply us with new wood to make new picks with to continue the excavation. I mean, part of the fun in building (for me) is the actual act of preparing to build. Digging, mining, finding ores, removing lava, exploring. If we are going to build an underground city like a Dwarven Fortress, then lets do it like Dwarfs do and mine it out ourselves. It's a multiplayer server, and a group project, lets get our picks working! Besides, a naturally mined cavern will be much more appealing and atmospheric than some squared off MCedit job.
That's all IMHO of course.
Oh, and I guess no one liked my idea of a city of towers on the ocean, lol.