Darcis surely won't be playing. It doesn't really change Belgium's chances in the doubles anyway, unless Desein/Bemelmans have useful credentials that I don't know about. Certainly Rochus isn't a strong doubles player.
If Hewitt plays well he can punish Rochus all day, yes...but if he is rusty and lethargic he'll be vulnerable, whether he has a dominant head-to-head or not.
As far as I read, Bemelmans will play the doubles if Darcis can't go. Steve actually is a decent doubles player. Bemelmans has a big serve and OK volleys, Olli is a good mover and decent volleyer also, although he doesn't have true doubles instincts. They actually are a decent team, IMO. I don't know about Hewitt/Hanley, I didn't think Hanley was that good at this point and Lleyton is likely still tired. I'm guessing he'll still play, and yes Australia would be the heavy favorite.
You are right about Lleyton v Rochus though, if Rusty is rusty, then Olli will take advantage of that. But I cannot see Lleyton losing that, not at home. If he's fit enough to give his all I think he wins. If it comes down to a fifth rubber things will get interesting so it's going to be a very important doubles point tomorrow.