You probably can add a firewall rule using the interface to do the same thing, the included proxy server contains a link to create reflect rules to put the proxy in transparent mode.
add 3000 fwd localhost tcp from ${server_ip} 80 to any in recv vmx1
# reload ipfw rules/sbin/ipfw -f /usr/local/etc/ipfw.rulesif [ -f /usr/local/etc/ipfw_custom.rules ]; then /sbin/ipfw -f /usr/local/etc/ipfw_custom.rulesfi
firewall_enable="{% if shapers or cp_zones %}YES{% else %}YES{% endif %}"