But how would that even work? Silvally uses the RKS discs to take on new Types, and it can't do that holding a mega stone.

Solution? Enter the RKS MK2 - mega silvally starts out rainbow-hued, and its Type and the appearance of its feathers and tail change as it uses an attack. It would work under the same mechanics as Pixelate or Gale Force, without the additional power boost.