I'm with you on this. I'd like to add weight to my tractor, but can't put it on the rear because I have a rear mower. Don't need it on the front. Don't want to fill the tires. Already have two weights on each of the wheels.
I have given some thought to finding several pieces of that 1" thick metal plate, like they use to cover holes in the street for this purpose. Not sure where to get something of that size that would be cut to order.
I also agree with and like your idea of using the MMM attaching system to hold the wieghts. Now, all one of us needs to do is build it.
Edit: Just had a thought (don't do that very often...) maybe make a thin block of concrete, like formed up with 2x4's, and cast the mounting points into the block that would mate up with the MMM mounting points.