So if I sac 6 {Shadowborn Apostle} can I search my library for {Withengar Unbound}?
Nope. Cards are always identified by their "sunny side up" side (or front)
So in your library it would only be an artifact equipment.
Here's a question, does the text on {Shadowborn Apostle} supersede EDH rules?
Going by {Relentless Rats} which wasn't allowed as more than a 1 of for edh (commander) I believe this would be the same.
So no.
Actually relentless rats is a legit strategy in edh, anythung written on a card supersedes the rules :)
Well then yes. If that is indeed the case! Lol