I haven't been here long enough to see how things work in the geocaching world so forgive me if I am talking a lot of nothing.
But, I am a member of a car club we have members from all over the world but keeping it to the UK we have the map divided into rough areas someone in each area volunteers to be a 'representative' for that area (usually voted in informally by people in that area) these representatives are then left to co ordinate meets in their area suggested by the members, we usually end up with a fairly small meet each weekend in various parts of the country which people are free to attend whether they come from that area or not.
We also visit car shows etc.
Now we have 'main' meets where the majority of events are held with the other meets being more of an informal gathering with people doing what they want during the day with usually a meeting at night for a few drinks.
I am not sure what marks a geocaching event as a 'mega event' but if you had people from different regions who were willing to hold and more importantly organise 'meets' then each area could take turns at organising a mega. It might sound a rather formal way of doing things but it could work out easier.
The trouble with organising events is you will always get some who say they will turn and for various reasons don't and you will get those that can't make ones some distance away and in this day and age with fuel prices etc being as they are people are becoming more careful over what fuel they use.
I take my hat off to anyone who organises any sort of meet, it is not easy and can be very stressful.
Like I say I haven't been here long enough to see how things work here and you may already do the above but just a few things to think about.
Sometimes a central committee can be helpful but it can lead to arguments as with anything and personal opinions can affect decisions, whereas the idea from someone can be posted for a 'mega' and then an online poll put up to see if it would attract the necessary numbers etc.