IGN has an interview with James Marsten (Spike on "Buffy", Brainiac on "Smallville") in which he discusses his role as Brainiac and the possible evolution of the character into Brainiac 5 of the Legion. Note that he doesn't say that he's going to portray Brainy, just that it'd be a different take on the character for him.
IGN: It's been funny on Smallville this season, because Brainiac has been a presence, but you haven't been a presence unfortunately.
Marsters: [Laughs] They're sick of paying me so much money. They were very generous about offering me some money up front and something told me, "Yes, we're going to work with you James for a good amount of time..." But if you notice, it's been like that a lot. They talk about the character, but... It's a good group of people, but I think that may be over.
IGN: Ah, so you don't think there's much of a possibility of returning?
Marsters: You know what, who knows? That series just seems to be immortal. It's done so well. So if the future is long, I'm certainly open to it. If they need me, I'd love to come say hi to everybody. But at the same time, they may not need me. They're very imaginative about how they do things.
IGN: The last time they had your character, in some form, they sent him off to the future to become Brianiac 5 – the good character from the Legion of the Superheroes.
Marsters: Oh, cool!
IGN: Yeah, it seemed like there was a door there for you to come back in a very different way.
Marsters: That's cool! [Laughs]
IGN: [Laughs] I might be throwing you a curve ball here, but would that be something that would be fun for you, to play such a different take on Brainiac?
Marsters: Oh yeah. I think that would be wonderful. I would also like to go back and be evil for them too. It's fun to be evil! It's fun to play a robot who doesn't really care and who's only pretending to. But it would be fun to play a robot who actually did care at the same time...