Change sources.list via Preseed

Greetings /r/linuxadmin!

I am playing with a PXE Multi-Install server in my lab and have it working to the point where I can do an unattended install of Ubuntu Server. My issue is that /etc/apt/sources.list is pointing to the PXE server, which is serving up the contents of the Ubuntu Server ISO and is not actually an apt cache, so running apt-get update on the newly installed server tosses up a bunch of errors.

Is there a way, via the preseed file, that I can change the contents of the sources.list to a local mirror, much like a regular CD install would do?

Thanks in advance.

EDIT: It looks like I can run a post-install script where I can replace the sources.list with one of my choosing/design via wget.

EDIT 2: This works! You just need to ensure to prefix your destination path with /target, as the installer mounts the new hard disk there.