Theoriticially it wouldnt be hard to set up, there are a few ways to do it.
1st, start off with 2 servers, each using the exact same database, each running 2 zones a piece. With mysql clustering/replication, you could keep these in sync, but this would be a bit slow, one acts as a master, the other a slave, so data updated on the slave might not be immediately available on the master. The better solution would be a 3rd log database that acts as a replicator, it gets posted the sql transactions on the other servers, which get immediately sent to the other servers in the clusters, so its pretty immediate in terms of data transfer. The mysql pipe is pretty thin, so a few mysql write transactions a second arent going to kill you in terms of bandwidth. Especially if you consider what changes in mysql, the data that is going to change is going to be player related mostly, so thats about the only data you need to pass around. You're not going to be passing around npc data, zone, items, etc, all that will be replicated initially when you set up the cluster. You could change those directly in the log server, and push it out to the servers. This kind of replication is pretty common.
Heres some information on a simple master/slave replication.
http://www.ls-l.net/mysql/
Its shouldnt be hard to set up, and it does have the benefit of allowing you to focus all your bandwidth on a single zone or two.
If the zoneserver is optimized well enough, it should be able to handle the load of 20+ players in a zone, but thats just an educated guess.
The hardest part would be getting the people together at the same time, and getting them organized. It's like herding cats...
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