A lot of metal bands did tracks that would slide in under the "progressive" door. Someone mentioned GFRR's "Closer to Home." Black Sabbath did quite a few: "Planet Caravan," "The Writ," "Symptom of the Universe," "Wheels of Confusion," and others that escape me for the moment. (Wakeman did session work on the Sabbath, Bloody Sabbath album, and I think Don Airey shows up on some later tracks as well.) There are a lot of other examples, I just can't think of any at the moment. Deep Purple did quite a few.
I was at a seminar about teaching English to middle schoolers when one of the presenters, a jolly, square-faced middle-aged lady in flowery polyester pants, began talking about using bands like Judas Priest to teach poetry (this was in the '80s). Her son was a big Priest/Maiden fan and she had heard enough of it to realize that there was a lot of solid lyrical content in the music. So here we are, a room full of teachers, most older than me and very straight-laced, listening to Rob Halford howl and grunt and talking about his lyrics. She was very sincere (and very good), but the whole situation just cracked me up. I've yet to sic Priest on my kids, but I've used a lot of ambient music as writing prompts with good success, especially some of the spacey, moody Eno material.