Okay, I've been toying around with this specific idea since last NecroWombiCon, in '03, and I though it might be somewhat feasible now.
Here's the idea - get a massive game of 40k going on one of the days, with points values reaching into the tens of thousands on each side. Armies organized into two factions (Order and Disorder from the last worldwide campaign works), and have a few objectives to capture a-la king of the hill. Just have one huge battlefront going for a few hours, not to mention the sheer size of the forces involved.
Now, the board sizes would have to be kept somewhat manageable (5-6' from side to side at most), with as much length as we'd need to deploy all the armies. It could take a good chunk of a day, but if it happened, it would be pure awesome.
Of course, there's always my original idea - same as above, but it's just Everyone vs. a huge horde of Orks. An Orkfest, if you will. It's a bit of a shot in the dark, but I think it could be mightily awesome.
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Realistically this would be difficult to oversee, and would occupy more resources than would be Feasible (I believe), Additionally the only 40K tournament will be 40K in 40 Min. due to the constraints of time and space.
If you could get players/attendees to bring the tables and Terrain then there may be space for this, if there is interest.
Quite simply, I'd post a call-to-arms over at The WAAAGH!. Could easily have a good 20k points worth of orks descend on the area, provided there's enough interest in it.
As for everything else, yeah, you raise a good point. All the other 'Orkfest' type events I've read about took more than a day to finish, with in excess of 40k points on each side. Still, it'd be freakin' awesome.