You could do a master/slave setup for MySQL.

http://dev.mysql.com/doc/refman/4.1/en/replication.html

All you had to worry about is redirecting all write requests to the master database. Either you have to program it into EQEmu itself or find/code a mysql proxy listener that knows which databases to send read and write requests to.
It must be determined how much bandwith such a setup requires on the uplink. Imagine some big come together event on a zoneserver that is on a remote location from the MySQL master database. But I have no idea how much updates per second per player must be written in the database on average and peak.

Matts stats are the ones that spook me a little. The servers we all own seem robust enough to handle plenty of running zones effectively - but he's right about the bottleneck at our cable modems or DSL. If our uplinks are not so great, once 2 groups of your players start tearing into something together, everything could run as smoothly as Morell-Thule in the early days. Anyone know what those servers hosting 30+ players runs on? must be more than DSL.

As for multi-honing our zones, sounds like an intriguing concept. I'd just be concerned with whomever owns one of the zone boxes staying on top of any issues that might arise, or losing that resource.