So, the big news in the gimbal world right now is the follow focus knob added to the new Zhiyun Smooth 4. Generally, I think the DJI products seem higher quality, but I am really being seduced by the idea of manual control for focus. Does anyone know if it is or will be possible to program the zoom switch on the osmo to control focus instead of zoom? Or, does anyone know if dji is considering adding a follow focus knob of some sort to the mobile phone line of gimbals?

If you get the Osmo Mobile 1 and purchase Filmic Pro (iOS) you can use the joystick to focus. (I suspect the Osmo Mobile 2 will also have this option once it is supported)
Works pretty well.
If you can make your own apps, you can program it like that. But not (yet) on the DJI Go app.
At least not that I have found.
But the SDK does allow for it, since Filmic Pro can.

Hi, thank you so much for your suggestion, I will forward this proposal to our engineers, hope more functions will be available for Osmo/Osmo mobile 2 in the future if possible.

Thanks for your feedback. Your suggestion will be forwarded.   

Yeah full manual focus control in FiLMiC Pro when using OSMO Mobile 2: you can see it demonstrated in that tutorial.

DJI Diana Posted at 2018-3-28 01:20
Hi, thank you so much for your suggestion, I will forward this proposal to our engineers, hope more functions will be available for Osmo/Osmo mobile 2 in the future if possible.

Hi , I was looking to buy Zhiyun Smooth 4 for this feature, Could you please check and update do you have this feature available in DJI Osmo 3. If yes, then I will buy DJI Osmo mobile 3.

thedudeabides Posted at 2018-5-9 14:01
Yeah full manual focus control in FiLMiC Pro when using OSMO Mobile 2: https://www.youtube.com/watch?v=vD5cxxE4rU8 you can see it demonstrated in that tutorial.

Hi , I was looking to buy Zhiyun Smooth 4 for this feature, Could you please check and update do you have this feature(in Youtube video @1:29) available in DJI Osmo 3. If yes, then I will buy DJI Osmo mobile 3.