I've seen the idea of retroactive funding for impactful projects tossed around a bit. An obvious case for this that comes to my mind is Eneasz Brodski's production of an audiobook for Harry Potter and the Methods of Rationality. I imagine that this audiobook has significantly increased the reach of the book. (And, personally, his audio production deserves most of the credit for why I consumed the book in entirety.)
Has such a retroactive grant been done? If so, I can't find any evidence of it.
update 1: Eneasz has confirmed that there has not been. Is there any reason why this might be a bad idea? This seems like low-hanging fruit to me, so I'm unsure why this hasn't been done.