The electrostatic energy in a capacitor is negative is it not? If one had enough positive and negative charge on the plates so that the electrostatic energy was greater in magnitude than the rest mass energy then the system as a whole would have a negative total energy. Would this be possible and how would such a system behave? In general relativity negative pressure behaves like negative gravitational mass. But if the capacitor is static then the negative electrostatic pressure must be balanced by the positive pressure in the structure of the capacitor so that the overall gravitational mass doesn't change. Maybe that's why the total mass/energy must remain constant as the capacitor is charged up?