...and that's why to me, logically, the OP is a home run. If a ball is a home run by passing out of the playing field touching only the top of the wall, then by the definition of home run, it left the playing field in flight. The definition of a ball in flight includes a ball that has touched a fielder. If the top of the wall doesn't render a ball no longer in flight, and we know touching a fielder doesn't, then touching both should not matter--whether it's fielder then top of fence, or top of fence then fielder.
If the issue is the vertical plane, then it doesn't matter what happens after it hits the top of the fence, and should be a home run at that point (and no balls on top should be considered in play.)
This is not how I would necessarily rule on the field were this situation ever to happen to me, however.