Does Shurelya's Reverie work on dead or decoy champions?
So the awesome GraceNote helped me do some research.
Kog'Maw's passive suicide form is not effected by Ghost or Shurelya's Reverie. However, its speed does ramp up until it explodes into the acidic goo.
Shaco and his hallucinations (and by extension Leblanc's decoy and Yorick's Ghost I believe) do gain the bonus from Shurelya's Reverie.