In case it helps I ended up with the following, which will send the rb5009-jailbreak.elf file. The machine it runs on is 198.168.100.1 on the local interface. Replace "-u username" with your username, put the file in /tftpboot/.
dnsmasq --bind-interfaces -i eth1 \
-a 192.168.100.1 --dhcp-range=192.168.100.100,192.168.100.200 \
--dhcp-authoritative \
--dhcp-boot=rb5009-jailbreak.elf \
--enable-tftp --tftp-root=/tftpboot/ \
-d -u username -p0 -K --log-dhcp --bootp-dynamic