[ChrisH]

I know that we have in the past added hardware that was legal (lead plate from SPI) but served no purpose other than moving the CG (Center of Gravity) of the robot.

Actually, it was amazing how a small shift could affect the handling of the robot.

Just be sure the material is tied down well, from a legal source, and either a kit part, a catalog part or on the additional hardware list. Since changing the CG affects the function of the robot, rules for non-functional materials do not apply.