The official Login Server source is locked down, but we do have an open source EQEmu Login Server available on the Google SVN here:
http://code.google.com/p/projecteqem...EmuLoginServer
It does everything that the Official EQEmu LS does and you can mess with the code all you like.
It already works fine with all of the clients that EQEmu currently supports including the current EQLive client. If you are considering trying to work on an older EQ client that isn't currently supported, then good luck, because without lots of packet collects from that particular client you will have an extremely hard time even getting the client to be able to log into the game much less make it functional enough to be playable.