Sorry I am still unsure here. Let me be clearer. Upon resolving Doomsday do I
1. Pull out 3 cards from library and 2 cards from graveyard as indicated on Doomsday and exile the rest
2. Can I put my graveyard (facing down) together with my library and pull out 5 cards while letting my opponent figure out the pile from the exiled cards?
The question here is more towards the physical action of resolving DD.
You can put all the cards face-down in one stack, shuffle, and make a face-down pile of 5 to be your library, then exile the rest. The question is, are the exiled cards left face-down, or face-up? If down, then you can keep your new library secret. If up, then as Duck pointed out, your opponent will just do process of elimination combined with what he knew was in your grave and figure it out.
Can any judge tell if the exiled cards face up or down?