There are two problems I have encountered creating a workaround to the lack of "External Controller" support in the PSR S.. series of keyboards.
1) The "External Controller" feature of the Tyros/Genos keyboards provides a mapping between midi Note/CC commands received and the assigned keyboard function, e.g., turn on "Talk" on receipt of a midi note C3.
Many of the keyboard functions (see Genos Reference manual pg 149-150) do not have documented internal midi sysex commands so it's not possible to create a external mapping capability to mimic the "External Controller" support of the Tyros/Genos keyboards. So there is an information gap here.
2) I am not aware of any generally available hardware midi controller which can be programmed to send mid sysex commands (even if the keyboard's internal midi sysex commands were known). These controllers normally send midi note/CC/PC commands not sysex commands.
As an experiment, I placed midi translation software between the controller and the keyboard (controller -> midi translation software -> keyboard) so that midi note/CC/PC messages are translated to the appropriate midi sysex commands for functions that are documented (e.g., Intros, Mains, Endings). This works, however, this still leaves a large number of keyboard functions of the PSR S keyboards which cannot be controlled by an external midi controller.
My purpose for adding this "External Controller" function to this wishlist thread is to have Yamaha spare the SX700/900 users (who are all not midi proficient) the effort of implementing a workaround which would still not be complete as the same function in the Tyros/Genos keyboards. A firmware update with this functionality would surely help those users with external midi controllers easily and better integrate with their keyboards.
There are many other features/functions of the Tyros/Genos keyboards which differentiate them from the PSR S line. I am not sure that this "External Controller" feature should be one of these differentiators, especially in 2020 but ..... I don't work for Yamaha's marketing dept :-)
I am not even sure that this forum/thread is the proper place for "Feature Requests" but it is said that Yamaha monitors these forums so here's hoping that they are listening. No harm in asking.
My other workaround would be to buy a Genos but I like being married :-)