Scott, I agree that sand moves in all dimensions at the particle level to dissipate system vibration as mechanical energy. The issue is somewhat different regarding drive train stability: Will the segmented top plates migrate or resonate in the horizontal plane in sympathy with transient pulling forces of the motor belt? Will any associated micro shifts in drive geometry introduce speed instability sufficient to off-set the virtue of improved isolation provided by a segmented top plate? One can only speculate on these matters until trying both arrangements with top-plates made from identical material.
Material composition is an important factor. For example, I first installed my DIY Stillpoints in cavities drilled inside the stock VPI delrin feet. Internal vibration was conducted from TT plinth to solid brass spacer plug(replacing stock sorbethane) to ceramic ball pyramid to spring steel bearing cup to delrin foot to wood to sand. Each modification was evaluated separately and was determined to be a sonic improvement. But the greatest improvement occurred after the final step of adding a brass core inside the delrin footer that directly couples the bottom of the ceramic ball bearing cup to the segmented hardwood top plate in the sand below. There is insufficient mass in this brass core to be significant as a vibration dump. Relative to delrin, the brass core must therefore be acting as an improved conductor of vibration down to the sand.
One might conclude that your aluminum top plate is performing a similar function as my brass core. Did you ever try the thick aluminum plate by itself to detemine if the plate is acting primarily as a dump, or as a superior conductor of vibration? If the sand adds any sonic improvement as compared to using the aluminum by itself, then one may conclude that the aluminum is performing as a conductor. And if the particular virtue of aluminum is its performance as a conductor, then one may speculate that your alum top plate also operates as a conductor of vibration in the horizontal plane between respective footers & motor. A superior conductor like an alum top plate might therefore benefit from segmentation more than other material compositions that tend to act as dumps. Segmentation may act to draw off the kind of "asynchronous" vibrations you alluded to above, that confuse a closed system where vibration is both generated & recirculated.
While it's nice to talk about reducing compliance as a absolute goal, I think this idea should be revisited in light of recent experience of jloveys and myself concerning the sonic virtues of Symposium rollerballs, Stillpoints, or my DIY version of same. These devices are designed to MOVE in horizontal, vertical, or both. I wanted to explore this with my recent TT experiment, and the results seem to suggest that mechanical systems like TTs need to "breathe" a bit to release energy. The result of this experiment is improved HF focus & smoothness, delineation of bass, and precise but rounded images-- anything but brittleness, softness or confusion.
