If you or anyone else is curious about more details on this issue and on how to set it up in general, you can find it here:
http://www.eqemulator.net/forums/showthread.php?t=25052
This problem is a pretty major one IMO and I would love to see it corrected at some point. In most cases though, the major limiting factor in server performance is upload speed.
The main reason I would love to have multiple zone servers is to get 1 or 2 of my heaviest zones moved to their own dedicated box just for them. I think it would increase stability on both PCs and help the server overall.
Honestly, to me, I don't think this would be a very hard thing to fix. But, I am not much of a coder (not at all really), so I can't do something about it myself or I would.
The sad thing is that everything is already there for this to work! All it needs is 1 or 2 seemingly minor changes to make it actually useful. If it ever does get fixed, I would gladly write a detailed wiki on how to set it up.