I had the same problem. I solved in another (very unclean and unsecure) way.
Just now I were looking around for some suggestion :-(
Anyway, this is my solution:
You keep in your LAN a PC with the fixed IP address and choose netmask and gateway
(eg 10.1.1.1/30 gw 10.1.1.2).
Assign the gw IP as the first address of the firewall LAN interface.
Assign to the same interface a second IP address for others LAN client, and configure firewall and nat rule accordingly (looking around you can find a step by step document about).
Create the tunnel as usual, then you can connect (only) from the PC to the remote LAN.
Ugly but working.
If someone have a better idea….