[megous]

You can also have public IPv6 address on all your LAN computers, and just open the firewall when needed. :D Even simpler.

Or even assign random unique public IPv6 address to your computer per individual customer, and listen on that address. That way, other customers will not accidentally hit the endpoints you don't want them to when demoing an app.

The same can be done for any other exposure of services on your LAN.