Their script is an alternative to applications (like this one). If you want to use port forwarding then you're free to implement what's required in an up/down script that's called when openvpn connects/disconnects. I don't have plans to add it natively.
Thanks a lot zomboided for the provided feedback - was just thinking if you came across this one. I've had it running previously with a custom script for port-forwarding. Unfortunately they changed the procedure entirely.
I will drop a PN just to ask you something - maybe you might be able to help.