How to reboot in %post?

Usually I don't want to reboot a built unit - I want to see the
final %post results and scroll back to check it.

But under some conditions I would like to reboot.  However, just
doing "reboot" doesn't work in %post (nor does shutdown -r now),
complains about no /dev/initctl.  In the Alt-F2 console I was
able to reboot via "mkfifo -m666 /dev/initctl; reboot" but that
doesn't work in the real %post.  I thought about somehow trying
to stuff Ctrl-Alt-Del into the keyboard buffer but don't know how
to do that.

How can I cause a reboot in %post?  This is for CentOS 5.6 at the
moment; would also want it for 5.5, Fedora 14, and others.