Manga News: Check out these new manga (5/11/15 - 5/17/15).
New Forums: Visit the new forums for Boku no Hero Academia!
Forum News: Cast your votes to determine the best parent in the Anime Showdown.
I was just thinking about that but it really is hard to tell whether they knew. I would find it rather strange if they didn't know though, they usually get their hands on information pretty quickly. Would they offer the position without having their fact straight? At least they would know rogue was his mom consider that would be easily verifiable. While I agree in that luffy will never accept the position I think the whole being offered a position is a different deal entirely. Luffy would make for a remarkably fearful shichibukai after all, his previous actions would see to that. Perhaps they could even go for a "the world government would rather become his ally than deal with him" angle.
Even if being offered the position would somehow contradict manga events that is exactly what being a shichibukai is about. You lend your services to the government in order to have your crimes erased. Shichibukai are pirates after all. Jinbe fought the marines on a daily basis, mihawk was shanks rival, flamingo controls the underworld, the kuja are actively pirating even when they have the title, kuma was known to be a ruthless pirate at some point and moria apparently went up against a yonko. Buggy (who has been mildly implied to be a shichibukai) was roger's former crewmate, best friends with shanks (as far as everyone knows) and took part in the wreckage of ID and marineford and he was still offered the position. If luffy is not offered the position then it is more likely to be because he would be uncontrollable even by shichibukai standards IMO.
It wont happen because the government is always going to see Luffy as an enemy. He's too strong, influential and uncontrollable, he'd never become their dog, ally himself with them or do anything that would make them change their mind, and he's a D and son of Dragon, so they'd always want him dead. If its like that, then none of Luffy's crew will ever get offered a position either, even though both Zoro and Sanji, perhaps even Robin, could all become strong enough for the position.
Anyway, it seems fame, or rather infamy, is just as important as strength, so Zoro is most likely of Luffy's crew, but as I said, as long as Luffy is an enemy of the World Government, so are the Straw Hats.
As far as we have seen none of the shichibukai are actually dogs. Most of them do not even cooperate with the government and can barely be bothered to even pay attention to it. Luffy is without a doubt strong but I doubt he is "too strong" for the world government to have him as a shichibukai. At the time he would seem to be even with jinbe and perhaps still weaker than the likes of hancock, kuma and mihawk. I doubt being uncontrollable would be an issue for the world government though. Its not like any of the other shichibukai are precisely cuddly or anything.