What you're trying to achieve isn't possible from my perspective. You have two options though:
1. Change the system and built a "customized" iso with remastersys you can afterwards boot without changes. remastersys is part of BT4 pre-final. Please search the web how to use it.
2. Customize the iso image within a chroot environment. muts provided a user-friendly script to do this over at the Offensive Security Blog.