Path Position is already exposed, and you animate it the same way you animate anything else from timeline:
- Create an animation track for you vcam (how? either create a new timeline for it, or drag it onto an existing timeline and select “Animation Track” from the popup that appears when you drop it).
- Hit Record on the new track (the little red button)
- Put the vcam in the inspector, and set a value for the path position, inside the “Tracked Dolly” section. This creates a keyframe at the current time (the field goes red to show that it’s being controlled by timeline)
- Drag the timeline Current Time bar to a different time.
- Set a different value in the Path Position field in the vcam inspector. This creates another keyframe at the new time.
- Repeat for as many keyframes as you like
- Stop recording.
- You now have an animated dolly.