According to convential current flow theory there would be no change in weight. As electrons piled onto the chaged plate (-), they would be drawn from the other plate in the capacitor (+), leaving 'holes'. At discharge, those electrons would flow back to their holes, creating a situation where there is no alteration in overall electron content between the two plates taken as a whole. As convential current flow theory is the basis of all solid state theory, it is plain to see that a charged capacitor will weigh the same at any state of charge. Anyone who disagrees with this obviously has no concept of electrical theory, says the guy who works as an electrical engineer and once built a functional lie detector as a school project. So nyah!
EDIT: Before you bother proving you're a moron by arguing with me, go read about the P-N junction.