Well, usernames are only hardcoded on the following files in the /etc directory:
passwd
shadow
group
So you could change the username of the superuser (root) account to something else by editing the references to those files. Or you could use the usermod command line utility instead.
While you can do this, it's a horrendously bad idea for the simple reason that bad stuff might happen.
Some applications are stupid enough to check the username rather than the UID, so they will not be able to match 'root' since you've changed it.
A better idea might be to create a different user and use sudo to give it the required root powers. Then you can use this account just the same as root.
