I get what you are trying to do - but the probability of success by doing this through MIDI velocity is limited.
- First - as you have experienced - some instruments don’t react to velocity at all (Hammond organ)
- Also, in many instruments, velocity controls not only the volume, but also other parameters of the sound (e.g. brightness, envelope duration)
IMO, you’d get more mileage out of working directly within the respective sound generator. In synthesizers, something like this is typically achieved by key scaling output volume (or modulating the amplitude curve with key scale).
For organs, take a look at Blue 3 - it allows you to fine-tune the volume of individual tonewheels and drawbars, which could help make that adaptation within Blue 3. But this will still be imperfect - turning individual drawbars or tonewheels up or down will change the spectral balance of the sound - it will not simply turn the sound of an individual note higher or lower in its totality.
But that’s also how things are on a real Hammond: you can’t just turn the volume lower on an individual key. Every key triggers multiple tonewheels, multiplied by drawbars; the only way to change volumes on a real Hammond would be to fiddle with the outputs of individual tonewheels or drawbars.
The only way I can think of to do this cleanly would be to use hammond patch in a VST sampler like Kontakt and edit the individual key zones within the sampler plugin. But of course, setting up a sample-based hammond patch is no mean feat. A static single sound is easy enough, but if you want to dynamically “play” the drawbars, things become a bit complex.
So no easy solution that I see.