Dont think it's gonna be Lex returning, sorry to say.
IF, and that's a big IF, Rosenbaum comes back, I could see him doing so for like 4 or 5 eps at season's end, to properly align his character in the mythos. I cant see him showing up in episode 4 and remaining throughout the next 17 episodes. He might as well have just signed on for the whole season at that point.
Unless of course they do an episode cliffhanger where it's like a 30 second shot of him doing something that doesnt require him to shave his head, and then we dont see him again until near ther season's end.
I follow Rosenbaum on Twitter and if he is doing something on Smallville he surely aint talking about it. He's filming some movie with Forrest Whitaker now.