I'd personally rather do a LARP than a massive PnP. With that many people, it seems like a PnP kinda breaks down.
That said, here are the major hurdles I think we'd have to consider for a PnP:
1) Temporal consistency: If there were a combat of any magnitude, we'd need to set a time-limit on how long a round can take (maybe 5 or 10 minutes). Otherwise, we run into the problem of "I shoot a fireball at the flying wizards overhead! What? They're already dead? Who killed them? Table 6? When did that happen? Round 15? We're on round 5 over here! ARG!"
2) Social interaction: If someone at my table wants to talk to the king, but the DM at table 4 is playing the king, do I have to get up and walk over there? Or does my DM take over the king for a while?
I think the best bet for a PnP game would be to have each table be a separate game, but all related to the same plot. For example:
The overarching plot is to prevent a war between two kingdoms, and each group is playing different interests in different locales. One group could be soldiers in the king's army; one group might be from the capitol's thieve's guild; one group could be emissaries from the druidic council. Each group would need to resolve a particular scenario, which would ultimately bring them together for a huge meeting/combat/whatever.
The DM's would be on some kind of schedule, too. By the end of the 1st hour, make sure the PC's have met NPC X, killed bad guys Y, and discovered clue Z. By the end of the second hour, they should be at location F, and know to talk to NPC H. Each DM would have different goals to complete by a certain time, just to make sure the group doesn't get too far behind.
The final hour would be the big get-together (it would probably be most fun to do a massive meeting, rather than a massive combat). Perhaps all the included groups would finally arrive along the border to discuss how they might stop the terrible war. This final meeting should be rules-light (so we don't have to deal with lots of dice and lots of DM interference).
I'm thinking it shouldn't be more than 30 or 40 people, divided into groups of 4-6 players each. Each DM would be in charge of creating the pre-gen characters for their group. The level cap should be pretty low (5th or 7th) in order to prevent too much unpredictable, plot-breaking magics ("I teleport into the castle and kill the king!")
That's my 2 cents.
I think The Kriegger Procedure would be easier to manage, and probably more fun. I got plenty of d20/D&D madness at the Con, and I think it'd be fun to do something a little different.
Spider