I've heard "Superstition" on quite a few classic rock stations, including some of the most uptight ones around (i.e. KQRS). I think it's a good choice for the format, since it breaks up the monotony a bit and throws a bit of a curveball to the audience. And I don't think anyone is complaining about it. It is a bonafide crossover track, and classic rock radio has always needed to play more black artists not named Jimi Hendrix (like Sly and the Family Stone).
I've also heard "Higher Ground" on classic rock radio.
I originally wrote that Jeff Beck plays guitar on "Superstition", but it appears that this is not the case. He did help to create the drum sound, and later recorded a cover version (Stevie Wonder initially offered the song to him, but Berry Gordy smelled a hit and insisted Stevie record it and offer Beck a different song instead). Also played elsewhere on the "Talking Book" album.
The song was also later covered by Stevie Ray Vaughan.