It is hard coded in the sources. I have disabled it on my install by making a small change in the source of user-list.vala.
A quick step by step guide.
sudo apt-get install build-essential apt-get source unity-greeter sudo apt-get build-dep unity-greeter cd unity-greeter-0.1.1/src/ vim user-list.vala +252
Remove the line or just add
// at the beginning of the line. Save and exit.
cd .. dpkg-buildpackage -rfakeroot -uc -b cd .. sudo dpkg -i unity-greeter_0.1.1-0ubuntu1_*.deb
Ubuntu will try to replace the custom built package when performing a system upgrade. Just uncheck unity-greeter to keep the custom one or reinstall the custom one using 'dpkg' after the upgrade.
To prevent the package manager from replacing your custom unity-greeter just run
sudo apt-mark hold unity-greeter