It might not be the best way, but I have
the same setup. What I do is to use the plan9.ini in the
CPU/auth server to set the ipaddress etc, so it can
boot from the file server and become the dhcp server.
Drop me a line if you want to get any detail about my particular
setup...

The file server does not run user processes, and therefore I
think it cannot be your dhcp server.

Another way would be to setup a different machine as a
dhcp server.