You have it covered mate, if you do all that before my fall grow, I won't have anything else to test. But I will anyway, at least as far as the simple comparison. Your comment on the round magnets is spot on, I will have to have another look at options. I have a drill press, so making an effective cage for the round magnets should be easy. If you set up for round magnets, I'd love to see pics of what you come up with.
I noticed one thing of particular interest on the KJ site, they showed illustrations of the magnetic flux density in various configurations, and much to my surprise the densest flux is not between two magnets attracting, it is above the surface of a single magnet with steel plates on both sides of it. It may be the case that the highest flux density would be sitting on the surface of a round magnet sitting in a fitted drilled hole in a steel plate. Sadly, I can't think of a way that we could determine that for sure. Maybe testing this setup vs. one with two identical magnets with seeds between them would be worth doing.
All these variations could use up a lot of seeds...