So, I noticed something odd earlier today. Our entire party was unable to get into a caravan (Time was running short when we entered, wasn't due to leechers for once), so those who got in decided to just drop and wait for the next one. When the next caravan came around, we noticed that a few of the chocobos, and some monsters from the old caravan, were still just hanging around. So, they most definitely have the capability for multiple caravans to run, there's no reason they can't instance it (As in, leve style instancing).

I tried to think of a method that would work for everyone, and came up with the following. First off, the party leader can sign up his entire party at once by talking to the NPC. If it's a party of 8, then that's it. They're signed up and just have to wait for their caravan to start. If it's not a full party, however, people may freely join. If your group is small enough, you may even just wind up joining another partially filled caravan. This way, people don't have to deal with people splitting up their party (Or worse, replacing a productive member with someone who's leeching or doesn't have a clue), yet at the same time keeps the more open style it currently has. In the interest of fairness, this should probably be applied to behests as well.