I think you're right about it being eqclient.ini, so I think you could just manually rotate that file.
Set all your key mappings on one character, then make a copy of eqclient.ini file for that char. Then set all your key mappings on a second character and do the same.
Now before you play just delete eqclient.ini and replace it with a copy of the character ini file, and you should be set. However, this will also give you a separate friends list and other preferences, and any changes you make to those friends/preferences would get lost unless you remembered to make a new copy of the eqclient.ini afterwards.
EDIT: You could probably write
a batch script (ie. a .bat file) that does this for you and then starts EQ (it might even be possible to have it always copy the file back after you're done, preserving your friends and other preferences).
Then you could just have one .bat file for each character, and use them to start EQ (saving you the trouble of manually rotating them).
I'd offer to write one for you but I'm on Linux and we have Bash scripts (which are different).