I would usually say I wanted to ask you, as if I had thought of it earlier and now I'm thinking about it again. Previously, before I saw you, I wanted to ask you that. A bit more casual I might say "oh, and I was gonna ask you". B is fine but it's a bit more direct and I don't think I've heard it as often if ever, but it's proper english. Nothing wrong with it. "Oh, also, I wanted to ask you… would you be at the political science club meeting Saturday?" doesn't sound right to me. The tense doesn't need to match because the question you're posing is a separate sentence from the introduction.