To configure it to send file and auth servers to plan9 machines, use a command line similar to the following.
dnsmasq -s some.domain \ --dhcp-option-force vendor:plan9,128,fsip \ --dhcp-option-force vendor:plan9,129,authip
Here, fsip and authip are the IP addresses of your file- and auth-servers. It would make sense to put these in /etc/hosts on the computer running dnsmasq.
It is necessary to 'force' the options since ip/ipconfig doesn't actually request these.