This is a script and LaunchDaemon that watches for the computer name field to be changed, then writes that to Munki's ClientIdentifier field.
Originally, I was overriding the HostName field (then using that to identify Macs to Munki), but Greg Neagle pointed out that this was not a great idea. This script is better!
You'll need Munki for this to actually be useful. If you want to build the pkg, put munkipkg in /usr/local/munki-pkg.
cd into the computer_name directory, then run munkiimport.sh to build the pkg and import it into your Munki repo.