Controlling volume with MIDI velocity

Well, an important question here is whether this is about adding a change in "volume" to the individual notes (e.g. global volume change on a per note basis) or if the "volume" is a metric of the number and intensity of the pipes speaking (e.g. a function of registration, like the crescendo shoe).  Both of which, the software can be somewhat manipulated into doing if running within a DAW.

In a some DAWs and VST hosts, you can remap the Velocity data to a MIDI controller/event (I'm not aware of any direct velocity mapping in OTQ but I could be wrong) that can then be mapped to OTQ's volume slider, the crescendo pedal (depending on if you want the organ volume or number of stops to change), or even an expression shoe (or three).  When I'm working in Dorico, for example, I can simply set dynamics as you would normally and use whatever "Velocity + [function]" expression map to change the registration quickly using the crescendo.  It's a bit of a hack, but it works surprisingly well.  In a situation like that, the change is global (e.g. hitting two notes on different manuals at the same time at different velocities will not be at different volumes and can even create unwanted Subito sounds), it can simulate something of what the effect would be like.  One way to minimize differences between manuals is to only have one manual mapped to the "Velocity +" expression map and keep the others at default, or set each manual to a different instance of OTQ that has its own mapping and only calculates values off of that one manual.  It's not perfect, but it would give a sense for how that could sound/behave.

While it's an interesting idea, I don't know how much it would really make playing better and more expressive.  However, on tracker organs, the attack velocity doesn't change the volume as much as it changes the timbre, since only partially pressing or slowly pressing a key will change the wind pressure and speed of saturation for the pipe, making the the transition between silence and "speech" for a pipe much more prominent--in a way that experienced organists can deliberately shape like an expression setting.  This can be a good or bad thing, depending on whether the player intended the effect, but I'd be very interested in more of that nuance being added as a function of velocity to OTQ.

For those interested in more specifics, here's a rather nerdy read on pipe speaking physics (albeit specifically ignoring the aforementioned effect but giving a good overview in general): https://newt.phys.unsw.edu.au/music/peo...al1983.pdf