Hi Ben,
I went into it because that was interesting for me.
In fact, in video mode locking the AF-C field is done via touchscreen. But at least for me the issue seems not not be an issue...
Usually I have the A6700 in stills-mode, AF-C, tracking field fixed middle. AF is superfast. Hitting on the "Video start" button usually starts with the focus distance from before. While taking video I am anyway not able to use joystick or similar to chose the subject. If focus is critical and difficult, I'd switch to manual focus. And you can select the stickyness of AF-tracking from 1 to 6...
Or did I not get your point?