What about a unranked mod that doesn't do anything, called Practice? It'd be great for playing those insanely difficult levels without worrying about your accuracy going down.
I've asked for this "friend" mod since a long time ago. It would allow me to easily let friends test when they watch behind my back and then just want to test a map or two. And that's just one use case, and there are more like the one presented by the OP. I really do think that this mod has more than enough uses to make it a good addition.