I think this card only works with Spirit of the Bat in the deck. A version with Keleseth might work okay, alongside the other generic handbuff card (Spiteful Warlock?) because at 3/3 Hir'eek is pretty good.
Does anybody know if the copies of Omega Agent also keep their buffs if it's hand-buffed?
Yes, it does. I was playing around with Hir'eek and Omega Agent yesterday to see if i could find some use for them. I made a control warlock deck that uses these two as alternate win conditions or as a way to pressure the opponent. It is okay but we need more support for this kind of strategy before it becomes truly viable for ladder. I made a comment about how this Hir'eek was a pointless legendary in this thread but if given the right amount of support it could become a powerhouse. For now, it's fun to mess around with it.