You can do pretty much anything with the EQ client so long as everyone using the server has the same files the server does.
i.e. new zones, monsters(with image files), etc.
I was involved on a very low level with hex editting back when TeamBG(
http://www.teambg.com) started mods on the Infinity Engine(the engine used in MANY Black Isle games, such as Baldur's Gate, Icewind Dale, Planescape: Torment, Fallout, etc.).
Anything is possible so long as you have the right resources.
- Xelnagan
- edit -
Fixed some typos... I was half asleep at the time...