I wonder if it will have the facility to trigger its organ externally over MIDI while playing its pianos from its own keys.
Absolutely, and the currently available models* also do.
The S9, S7 and J7 all share the same engine; on the S series, plug a suitably weighted class-compliant USB MIDI controller (or use old-school MIDI) and presto, you're playing the organ. On the J7, do the same with an 88 weighted controller, and voila, the same pianos as in the S (some variations may exist in the editing, check the manual for details before jumping to any conclusions or otherwise quoting me on this).
* in these posts I am only referring to the S9, S7 and J7, not any other model, so please nobody make any silly statements that one of their other models sucks because it doesn't do this, or any other such stuff that often pops up around these parts lately.