Going to the next discipline, if you're in the park doing spin tricks, weight on your body would be close to the centre of gravity, while weight in your board/bindings/boots would be much further out. Watch a figure skater spin, and when they pull their arms in (i.e. all the weight coming towards the centre) they actually accelerate without putting any more energy into the spin itself. This is why even identical model boards will be much easier to spin if one's shorter, all the additional weight is out at the tips.
Totally agree with that - but, as you said it is a completely different point. And you are slightly incorrect that it is about the center of gravity - what matters is the axis of rotation.
The ankle weights will make spinning much more difficult, because they are further away from the axis of rotation which (unless you are doing inverts or corks) generally is line with your spine. The vertical positioning (board level, chest level) does not really matter.
To make the suggested experiment with the weights relevant, the weights would have to be either i) moved in from the ankles, closer to the center of the board (roughly in hip width) or ii) further out from the hoodie pockets or backpack. I contend that if you did that the vertical positioning of the weights (board level vs. chest level) would not affect spin performance.
Actually this leads into another theory of mine, the extra wide (24+") stances that some guys ride with now actually hurt their park performance. The bindings, boots, and your lower legs are that much further apart making it take more energy to spin at the same spin speed.
Completely agree with David's response to this - it is pretty much accepted that a very wide stance reduces the ability to spin and pivot the board.