Elliot, The spring suspension isolates the chassis, motor, platter, bearing, etc, from the plinth, at least that is my understanding without having owned an LP12. So I would think of the plinth as being outside the suspension system. One would have to be careful adding further isolation that affects the plinth, since any additional external suspension might spuriously interact with the springs, producing oscillation in the worst case scenario.
Incidentally, I own a Denon DP80 that came to me with the DK300 plinth and a Victor TT101 that came to me with its OEM plinth. I ditched the DK300 in favor of slate, and I heavily modified the TT101 plinth by (1) replaced the armboard with a solid aluminum copy to which I added mass and further rigidity from below, and (2) added about 20 lbs of mass and constrained layer damping to the plinth by bolting to it some large pieces of solid aluminum, from below. The OEM TT101 armboard was a joke in the light of our current understanding of the need for rigidity, mass, and coupling to the bearing. I can't swear to it now, but I think the DK300 is made from MDF, which to my ear adds an obvious cardboardy coloration.
