I take control of my opponent's Valyrian's Crew that already has some facedown cards attached to it. Naturally, I do not take control of the facedown cards, as with any other attachments in play.
Valyrian's Crew says I can play attached cards as if they were in my hand.
RRG "Facedown Cards" says "A player may look at the identity of any facedown card that is in play and under his or her control".
I do not control these facedown cards, so I cannot look at them, but I can play them as if they were in my hand... So how does this work?
You may marshal or play facedown cards attached to Valyrian's Crew as if they were in your hand.
Reaction: After you win a challenge in which Valyrian's Crew is attacking, attach the top card of the losing opponent's deck to Valyrian's Crew facedown under your control, as an attachment with terminal.