You're going to not want the dock to zoom out if you decide to do it this way (I'd choose another action like rocking or something), but here's one way to do it. Experiment with how big you want your dock set at. Then once, you know the size you want the dock to be for your liking, take the image you use as an indicator and edit the PNG file so that the width is the same and you have a much larger height.

Make sure the indicator image stays on top of this new canvas and the new blank space where you added height stretches downward (this seems the opposite direction of where you might be inclined to put the indicator, but trust me). You'll probably need to experiment with the size of the blank space depending on how big you want your dock.

The icons themselves don't activate this feature. Objectdock Plus does when one of the programs is being used.

Top Dock


Bottom Dock


In each pic, you can see the edge of the dock. I want the top dock's indicator to be on the bottom of the icons just like the bottom dock's. Perhaps OD+ adds the indicator on the border of the screen? Like if my dock were on the right or left, the indicator would be to the outside of the screen.

Ok, not really good at working with image editors or Photoshop, so I guess I'll have to live with it.

Since the one image for the indicator is used for all 4 screen edges, when it is designed as needed the indicator will display at the screen edge of the dock on all positions.