I added Dynamat to the underside of my Rondine Jr. platter. It did very little to lower the noise floor, more noticeable was how it took some of the edge off the 'table's sonic liveliness. I'm going to clean it off soon. It was a waste of time and material.
I've learned from applying Dynamat to this table as well as a Rega Planar i used to have and a couple older DDs that it's easy for me to kill the life of a 'table by adding too much damping. (adding the prescribed postcard size square of Dynamat to the underside of the Rega really, really dulled its overall character. )
As for adding weight to the platter, that will increase the load and therefore the friction on the ball bearing at the bottom of the main bearing well which could actually mill a divot into the bottom of the shaft increasing contact area further increasing friction (and noise). I'm also not sure what effect you're going for by adding that weight; since this 'table is idler driven, it doesn't rely on inertia for speed stability.
I can appreciate that these 'tables can make fun and engaging playgrounds and no harm done if everything is reversible (although I might be concerned about making a dimple in the bottom of the main bearing shaft w/the added platter weight), but I've found that simply restoring my Rondine L-37 back to original specs (new motor mount grommets and idler wheel rubber, etc) and mounting it in a solid plinth leaves little desire for improvement.
