How to setup DMZ over OpenVPN on OpenWRT/Gargoyle/DD-WRT?

Question

openvpn

After setting up OpenVPN on a router with Gargoyle firmware, DMZ stopped working.
You must either add or edit a rule in one of the chains to wrap incoming VPN traffic to a specific IP address.
Contents of iptables in terms of incoming VPN connections:

Chain zone_vpn_dest_ACCEPT (3 references)
num  target     prot opt source               destination         
1    ACCEPT     all  --  anywhere             anywhere            

Chain zone_vpn_forward (1 references)
num  target     prot opt source               destination         
1    forwarding_vpn_rule  all  --  anywhere             anywhere             /* user chain for forwarding */
2    zone_lan_dest_ACCEPT  all  --  anywhere             anywhere             /* forwarding vpn -> lan */
3    ACCEPT     all  --  anywhere             anywhere             ctstate DNAT /* Accept port forwards */
4    zone_vpn_dest_ACCEPT  all  --  anywhere             anywhere            

Chain zone_vpn_input (1 references)
num  target     prot opt source               destination         
1    input_vpn_rule  all  --  anywhere             anywhere             /* user chain for input */
2    ACCEPT     all  --  anywhere             anywhere             ctstate DNAT /* Accept port redirections */
3    zone_vpn_src_ACCEPT  all  --  anywhere             anywhere