Today I needed to update the default administrator password in an SCCM Windows 7 Operating system deployment task sequence. It took a bit to find it.
To change this, edit the task sequence. Expand PostInstall -> Apply Windows Settings -> Enable the account and specify the local administrator password.
Note: The best practice is to set a random password and disable the account during the OSD. Then you can do what you want with the account via GPO after it joins the domain. This customer isn't following that for this particular sequence because it is the reference image. They want to log in and check the reference image before pulling the WIM file.