Hi Uli --
Hope this posting makes sense...
I had a chance to experiment this afternoon with drawbar effects. Bottom line: I see what you mean.
My foot pedals assignments are: 1. Sustain, 2. Rotary speed, 3. Volume. I monitored MIDI A OUT with Midi Ox and verified that the expression pedal on foot pedal #3 is sending MIDI CC11 (Expression). The foot pedal changes the volume of the entire channel; yep, it's at the end of the tone generator/effects chain. Not what we want if we want the organ level to affect the amount of distortion in a distortion block somewhere in the effects chain.
So, I tried an experiment. Select a drawbar (organ flutes) voice in RIGHT1. Replace the rotary effect with a distortion effect. I decided to use Y-AMP British Crunch. Now adjust the Volume knob in the drawbar Voice Edit display by "turning" the faux volume knob. The amount of distortion tracks the organ volume level. So, yeah, Y-AMP distortion behaves like real electronics. Setting the Volume knob to 3 is clear, setting to 8 is fuzzed out bliss.
Then, spend 10 minutes playing You Keep Me Hanging On by Vanilla Fudge. Whoops, sorry, that did happen.
Next, replace the Chorus effect with Dual Rotary Speaker Warm. Adjust the Chorus send level for RIGHT1 in the Mixer Chorus/Reverb page. The signal chain is:
[RIGHT1] [INSERT] [CHORUS] [REVERB]
Organ flutes --> Y-AMP --> Dual Rotary Speaker Warm --> Reverb --> Out
Open the Dual Rotary Speaker Warm effect and use the on-screen SLOW/FAST switch to change the rotary speaker speed.
In some fashion, the signal chain can be kludged up, but the controllability is horrible! Neither organ volume or rotary speed can be adjusted by assignable knob/pedal; this must be done in the appropriate screen. Changing the insert effect to Y-AMP leaves the rotary speed switch, etc. dead (unresponsive) in the skeuomorphic drawbar Voice Edit page.
Not really a solution, just a possibility. Ideally, Yamaha would rework the drawbar organ interface and signal chain to allow a distortion block before the rotary speaker effect and to let the foot pedal adjust the output from the drawbar emulator to the distortion block. Software enhancement?
All the best -- pj
P.S. I tried similar experiments with sampled organ, too.