In looking through the GPL source code for Yamaha's synthesizers, I discovered that the Motif XF (and the Motif XF alone), as of version 1.40, appears to have a VNC Server installed. The source packages are LibVNCServer-0.9.7 and ymh-libvncserver-0.9.7-motif_xsmc.1.

For those who don't know, VNC is used for viewing one computer's screen from another over a network connection, as well as potentially interacting with it.

At least, the source is included in Yamaha's GPL compliance package. They even went through the trouble to patch it themselves in order to make remote keyboard commands functional (it seems). It may be the case that this package was only installed in development or testing builds of the XF's operating system. It could also be the case that the server is installed in production hardware but does not run by default, requiring some unknown signal or key sequence to activate.

As a bare minimum sort of investigation, someone with an XF could try connecting to the XF using a free VNC client in order to see if it is perchance running and listening on the standard port.

To see if it is running on a nonstandard port, one could run an nmap scan (the included Zenmap GUI interface is easy to use) in order to see if the XF has any open ports besides the expected ones (5555 & 5556 for the network audio feature, as well as 139 & 445 when the File Server is turned on).

Given that Yamaha hasn't advertised this, if it even exists in XFs out in the wild, the standard disclaimer applies: this is an internal testing tool and, like the Reface CP hidden piano, may not meet the standards of functionality on which Yamaha prides themselves.