My first thought was "I bet getting ntttcp on x86 with Ubuntu/CentOS is ok, but I doubt it will be simple for something like OpenWRT."

    root@router:~# opkg find ntttcp
    root@router:~# opkg install ntttcp
    Unknown package 'ntttcp'.
    Collected errors:
     * opkg_install_cmd: Cannot install package ntttcp.
    root@router:~#

Microsoft would be so much better served putting resources into proper Windows support for iperf3, instead of creating their own tools and convincing the world to switch.