Comment 1 for bug 782007

As a workaround for my own purpose, I've modified the execscript to be run later by just moving the call to self.callhooks('post_install') in Distro to self.distro.call_hooks('post_install) at the end of install_os, before unmount_partitions.
  With that change, I seem to have much more control to achieve what I want with my execscript.

I'm a little new to launchpad, but I'll try to post a patch shortly to demonstrate the change.