I went straight to rock because post-rock is most relevant to us and likely the source of all of our "post-" metal genres, but yeah, you can apply it (and it has been applied) to any style that progresses.
But you do have stuff like post-minimalism like Grenn Branca or post-industrial like Psychic TV or post-punk like Joy Division, all predating post-rock.
Post-rock isn't really the first "post-" genre. The "post-" prefix is usually used for music that breaks out of the confines of an original genre, usually with different structures, but it's pretty arbitrarily used.
