The routers I have used do require static IP to map port forwarding when you want to be able to access the device from the outside. Nevertheless, the place to set the static IP is right there in the router while setting up the port forward. That way, if you change the port froward, you can make the corresponding change to the target IP at the same time.
You are correct that to do port forwarding you have to have a static IP address. But, doing a port forward is probably not a great idea with today’s threats. Not sure exactly what damage somebody could do by accessing an esp8266 based device. There are valid reasons for needing or even wanting a static IP address. I have memorized the addresses of my two devices, so not hard for me to type them. But names are easier.
Yes, this works very well on my Win7Pro64 PC. Yesterday when I rejigged my routers and WattsOn’s IP changed from the DHCP-assigned …130 to the MAC-bound …123 I was able to immediately click my wattson.local bookmark in Firefox and it instantly found the Configuration Menu.
Daniel
I think the relevant clause there is “Decades ago”. Technology has changed massively in that time. Today, I can ask a customer to install Ping Tools (free) on their Android phone, run a Local-Area Network scan to list the devices (with IP addresses) on their LAN. Sort them by address and look at the range of addresses excluding the router’s (almost always the lowest). Armed with that information and further instructions, they can be educated on how to choose an appropriate new IP address to fix [e.g. either well below the starting range of DHCP-assigned addresses if they start high (like mine), or well above the last found address if they start low (like on my friend’s router)].
Daniel
In any event, my commercial model (if I decide to proceed down that path) envisages WattsOn systems being installed by a trained installer (probably the electrician required to install the solid-core CTs), who will also do the initial configuration. So the customer can remain ignorant about the inner workings of IP addresses and such. 
Daniel
While technology has changed massively, people’s knowledge of the intricacies (or even basics) of networking has not really improved massively. However, I am sure you know your (potential) customers and/or installers better than I do. So, hopefully they will be as capable of doing what you need as you think they are.
I do occasionally still visit various places where people struggle with networking. And yes, it is reasonably easy to tell someone to run some tool. But for many cases, with many people, the results will not be as expected and then the fun begins.