I never really had any need or intent to use my WK7500 as a controller, but I thought I would give it a try so I could see what you have been going through. I used Sonar and Ableton Live, but sad to say, I got about the same results you did with FL10. First of all, I assume you have figured out by now that the Perc and Rotary buttons are only active, internally or externally, when the keyboard is in Drawbar Organ mode. When it is not in Drawbar mode, those buttons are dead to the world. In Drawbar mode, when I would press the Rotary button, I got no response from Sonar at all, even though the MIDI-IN monitor light on the PC would flicker. So, I knew the 7500 was sending something. With Ableton, whatever voice I had selected would momentarily experience a slight change in timbre and volume when I pressed the Rotary button, but that is all. It's like Ableton did not know what to do with the data it was receiving. I got no response at all from anything when I moved the Drawbar sliders. I didn't even see the MIDI-IN monitor light flicker, so I suspect the 7500 was not even sending anything from those. It would appear that the DAW software assumes that you are using a programmable controller that you have programmed to send the correct code for whatever the DAW software is looking for, but the 7500, being a keyboard and not a programmable controller can only send what it was designed to send, So, the 7500 sends one thing, the DAW is looking for something else, and never the twain shall meet. I take it from the questions and frustrations I see being expressed on various forums - how do I get my controller to do this and I can't get my controller to do that - that these kind of setups are not all that easy, even if you do have a full fledged controller. Those that seem to have the most success are those that have DAW software that came with controller setup templates, one of which matches the make and model of the controller they are using.
So, for now, I am going to withdraw from this and go back to playing and enjoying my keyboard.
Best of luck with this, JRH. Wish I could have been of more help !