I just got it to work in a slightly different way. I can probably delete my NAT rule as you surmised so I'll play with it a bit, but I wonder if it's more secure keeping the NAT as it has to follow a traditional port-forward-nat rule first.
Basically the gist of it is I point it to my internal pfSense LAN IP and I assume STunnel does the rest.
Stunnel rule
Listen on 192.168.1.1 (internal IP of pfSense firewall LAN)
Listen on port 3456
Redirect to 192.168.1.15 (Camera software box)
Redirects on port 81
NAT rule
Interface WAN
Protocol TCP
Dest Address WAN Address
Dest Ports 3456
NAT IP 192.168.1.1
NAT Ports 3456
NAT created FW rule
Protocol IPv4 TCP
Source *
Destination 192.168.1.1
Port 3456