Andrew Friggin’ Kramer did this
If you have particular, use the technique up there. It wouldn’t be a “line” it’d be so many circles so close together that it would look like a line.
If you don’t have Particular, peep this:
1. Draw a mask on Layer 1, click on the Mask Shape property, copy to the clipboard.
2. Make a Layer 2. Create a new solid the size of your dot (30×30). Double click the Eliptical mask tool (this masks out a circle).
3. Press P. SELECT the position property (by clicking on it). Paste the mask to the position property. You should see two keyframes and a bunch of roving keys in between them. Your dot should now animate along the mask shape. Hold down option while you drag the later keyframe to expand the animation (to make it slower or faster).
4. Go back to Layer 1, apply 3D Stroke. Animate the end from 0 to 100 at the same keyframes as the position for your ball.
see the demo