Rega spent a huge amount of time reducing weight and optimizing the table for light weight to reduce resonance. From my experience when you add weight you take away from sound quality. This goes for what the player is sitting on as well. you do not want to add mass to a Rega playback system. I also do not think the plater is optimized for a clamp or weight either, but if you NEED one use something, a light weight clamp over a weight.
We did an experiment with the rega table sitting on a hi mass shelf and a low mass shelf and it was very aperient that the light weight shelf was superior in all regards. this was a Rega balsa wood shelf that weighed very little.