1. Get a copy of Mountain Lion, I got mine from Apple. If you're not a dev, maybe you can get it at "the bay full of pirates"
2. Mount the Mountain Lion DMG installer, right click on OS X Mountain Lion Preview 1.app and click "show all content" Navigate to Shared Support and drag "InstallESD.dmg" to your desktop.
3. Bring up Terminal, (spotlight search Terminal) type in *remove $*
Code:
$cd desktop
$hdiutil convert InstallESD.dmg -format UDRW -o InstallESDhacked.dmg
After the process is complete, you will see a "InstallESDhacked.dmg on your desktop.
This will allow us to modify the new installer (read/write)