Most of the debs Ubuntu uses are installed into a chroot before generating the ISO image, so you can't just get them back. If you unsquash the filesystem, you'll see it looks like just a root filesystem. For example, this is a truncated view of ls:

drwxr-xr-x   2 root root  4096 Oct 12 16:59 bin

drwxr-xr-x   3 root root  4096 Oct 12 16:59 boot

drwxr-xr-x   4 root root  4096 Oct 12 16:56 dev

drwxr-xr-x 134 root root 12288 Oct 12 16:59 etc

drwxr-xr-x   2 root root  4096 Oct  9 08:59 home

...

If you want to mess with the chroot, you can do so as follows:

1. Mount the ISO

   
   
   

   sudo mount -o loop quantal-desktop-i386.iso tmpiso
   
   




2. Copy the squashfs out

   
   
   

   cp tmpiso/q/casper/filesystem.squashfs /tmp
   
   




3. Unsquash the filesystem

   
   
   

   sudo unsquashfs filesystem.squashfs
   
   




4. This will unpack the filesystem into squashfs-root.



5. Make your changes.


6. Recompress with squashfs.


7. Use ISO master to replace the filesystem.squashfs with the one you modified.


8. Rebuild the ISO with ISO master.