http://www.advogato.org/person/thomasvs/diary.html?start=10

Now, %post executes in a chroot'ed environment equivalent to your newly installed system. So there's no way to get at the file you want to install. But you can run %post without the chroot'ed environment by using —nochroot. Only problem is, you cannot go back to the chroot'ed environment (I thought the chroot command might do the trick, but that either starts a new shell interactively, which is bad of course, or allows you to execute the one command you put behind it).

So what do you do when none of the solutions is aesthetically pleasing, and a little voice in the back of your head says : Wouldn't it be nice if you could just have two %post sections, one labeled "%post" and another labeled "%post —nochroot" and just put the right commands in the right section ?

And you think to yourself, yeah, that would be nice, but no way would that work. I'm sure the Red Hat engineers didn't code to allow that, and otherwise I would have come across that on Usenet and in the docs.