Respectfully, sir, I would encourage you to read your manual again on this matter. Although you're assumption is correct that you can't assign the volume or presence slider to transmit midi data, the JV90's C1 slider can indeed be assigned to transmit controller data internally, externally, or both. Furthermore, you can actually use the 8 parameter sliders to transmit data while in V-Exp mode; although (as far as I've learned) it would be limited to the 8 pre-assigned parameters (left of the sliders), and at that, at a very poor resolution speed, so I wouldn't recommend their use for quick/smooth filter-like fx.
Apologies for derailing this thread.