And the best way for me, as GM/designer, to hide the truth from them is to keep it secret from myself

I don't think I could do that. I'd want to know. Even though I'd be quite happy to hide it from my characters (in the case of a story), I'd still want to know, in order to make the "universe" behave consistently. But maybe that just reflects my metaphysical assumption that any universe, real or imagined, doesn't change its "laws" at random.