Progress

A driver that can be installed, recognized by macOS, and loaded by the operating system
A driver host app that installs the driver, connects to the driver's user service, and displays a virtual controller
An emulated NES controller that is usable from the host app
Apps such as OpenEmu recognize the emulated device as a working HID game controller
iPhone app displays the emulated NES controller and connects to the driver
iPhone app sends button state to the driver

Progress · SillyUtility/VirtualController Wiki · GitHub