Derrek wrote:
From what I understand of the organ and the way manuals and combination pistons work, your really need a player that lets you change the sound-slot assignments’ MIDI channels via CC numbers.
for all intents and purposes one needs to take the stops in separate slots and say put the right hand staff on MIDI 01 and then send controller codes that tell the player “set slots 1,4,5,10,17 all to receive on MIDI Channel 01.”
To play a tone on an organ, one needs to choose its divisions (e.g., manual I), engage the intended stops on that division, plus triggering the actual note. If one allocates a separate (grand) stave for each division, then one could allocate each division a fixed MIDI channel and each staff a coresponding MIDI channel and then communicate via that MIDI channel for sending all the other information concerning that division.
Usually, organ music is only notated with 3 staves and therefore the division(s) allocated to a stave can change during a piece. You are suggesting to do that by changing the MIDI channel(s) of the responding divisions on the fly. Even if that would be technically possible, doing so would result into problems as soon as you use multiple divisions per stave with overlaps in terms of divisions between staves and then want to change the divisions again as it could result in conflicts between staves.
Instead, the free virtual organ GrandOrgue offers an abstraction that provides a way around, which in GrandOrgue has the fancy name coupler coupling manuals. The concept is straightforward: think of them as an additional set of manuals and pedal, but which by default have no division at all assigned to them. Normally, in an organ, each manual/pedal has its own division (great, swell…) and further divisions can be coupled to it, but its default division cannot be removed. Instead, a coupler coupling manual has no default division.
In my setup (already working in Sibelius), each organ staff sends MIDI on a specific channel (three channels in total), and for each channel one separate coupler coupling manual listens to it. Arbitrary division combinations can now be assigned to each organ staff without switching MIDI channels, by coupling arbitrary divisions to its coupler coupling manual via certain MIDI events. That setup allows to express all division combinations possible on an organ (as long as thumbing down across manuals and divided pedals are ignored).
I don’t know the commercial virtual organ Hauptwerk well enough, but I would be surprised if it does not offer a very similar feature, and the recently released Organteq might provide that in time as well (if it is not already there).
I don’t envy the challenges ahead in this area
I think the actual challenge is that every organ is different; they all offer their unique set of stops etc. Therefore, if one wants to engage specific stops explicitly in the score by playing techniques, for every organ and every stop of that organ, a playing technique and its corresponding playback settings has to be customly defined.
For my own purposes I try to reduce this problem by using another abstraction layer, stop combinations defined by the virtual organ. GrandOrgue offers for each of its virtual organs not only a 1000-slot combination setter (where each stores the complete state of the organ), but also 10 divisional combination setters for each division (where each stores the engaged stops of its division). I am not using the global combinations, but instead the divisional combinations, because they are more flexible (together they offer an even greater range of stop combinations), but I also find them more easy conceptually. They are less setters overall, and only the, well, combinations of these combinations result in the intended flexibility. So, I define a custom Dorico playing technique for each of the divisional combinations (40 in total for 4 divisions: 3 manuals + pedal) only once, and then can change at the virtual organ conveniently what stops these combinations amount to. For better memorability I may arrange, e.g., the combinations for each division from very soft (combination 1) to loud (10), but might additionally arrange them depending on timbre, e.g., combinations involving reeds only on even combination numbers etc.
I only need to define this setup once in Dorico, and when I change the organ (or want to change the overall registering on the same organ) I just need to allocate divisional combinations at the new virtual organ. It is a compromise, of course, but much preferrable to me than instead defining a separate playing technique for each stop for each organ in Dorico (leave aside that technically that is currently not supported, while using combinations works, as their are not multiple playing techniques – stops – engaged “in parallel”).
Now, my currently outstanding problem in Dorico is that I cannot have an independent control over divisions and stop combinations on a single stave (see discussion above), but as at least a temporary work around I am considering to use a separate stave for each division.
rubberfingers wrote
Anyway, I’m sure Dorico in a few years will have something pretty cool to use.
Considering the care they demonstrate in the Dorico design so far (e.g., just the recently introduced harp pedalling), I am very confident the Dorico team will find a good solution at some stage 
