variable-speed focus PTZ protocol
Looking at this (unofficial) Pelco D/P tutorial there are message types to set the zoom/focus speed in the extended command set.

I've used wiper/washer/lamp and cam presets features on a Dedicated Micro DS2 controller before. You could always hijack those to send increment/decrement signals to a slew rate parameter tracked at the housing end. This might be useful if you want to use an existing control joystick unit that doesn't support the extended set. 

You could try signing the NDA to get the official docs to confirm.