I think "played straight" is the wrong word in any sense for the Ginyu Force. They're ridiculous characters - that's their shtick. They're just ridiculous for different reasons in different hemispheres, which is alright by me. Americanization isn't a bad thing.
That's because A) Spain doesn't exist in the One Piece world and B) as far as I know, there's no way to indicate the specfic way a Spanish person would be speaking Japanese. If the show was created in the United States - which it would be but, work with me here - he'd probably be Spanish. He looks Spanish, he has syrabols evokative of Spanish culture, and giving him an accent - not a ridiculous, over-the-top accent, mind you, but a subdued and realistic one - would only give more depth to that portion of his characterization.
And I wouldn't mind Zoro with a Japanese accent. Well, I would, because as you said, Sabat doesn't seem able to pull off a serious accent, but if it was an actor who could, that would actually fit. Same with with Sanji - a slight twinge of French wouldn't hurt him at all. He's a chef, a smoker, a womanizer... associating him with France, which itself is associated with all of these things, only deepens the understanding of the American audience in a way that you simply can't get in Japan.
It's the same with with DBZ. Like, Dr. Gero - if I was in charge, I probably would have given him a German accent. Maybe give Commander Red an Irish quality to it. Nam has an Indian accent because he's Indian. American culture is VERY different from Japanese culture, and it both widens the audience and increases American understanding to touch up the show with these things.
Not paint over, mind you, but touch up. When you remove the voice acting and need to alter the dialogue, there's a big blank chunk of canvas to work with. I simply prefer it when the artist adRAB their own touches.