an even earlier post-rock

David Griffiths spots an appearance of "post-rock" even earlier than Ellen Willis in 1968 -- and in a Time magazine September 22 1967 cover story on The Beatles of all places. In "Pop Music: The Messengers"Time staff writer Christopher Porterfield says:

Rich and secure enough to go on repeating themselves —or to do nothing at all—they have exercised a compulsion for growth, change and experimentation. Messengers from beyond rock 'n' roll, they are creating the most original, expressive and musically interesting sounds being heard in pop music. They are leading an evolution in which the best of current post-rock sounds are becoming something that pop music has never been before: an art form. "Serious musicians" are listening to them and marking their work as a historic departure in the progress of music—any music.

Later in the piece there's a subsection titled "Sound Pictures", and the quotes from George Martin anticipate Eno and studio-as-compositional-tool:

George Martin, the producer whose technical midwifery is helping to make the steps possible, likens them to the shift from representational painting to abstractionism. "Until recently," he says, "the aim has been to reproduce sounds as realistically as possible. Now we are working with pure sound. We are building sound pictures."

In fact, some observers predict that "sound pictures" may prove to be the medium through which the Beatles—and the more adventurous rock groups in their wake—can merge with "classical" contemporary music. Already, says Robert Tusler, who teaches 20th century music at U.C.L.A., "the Beatles have taken over many of the electronic concepts in music that have been worked on by the German composers of the Cologne group. They've made an enormous contribution to electronic music".

So post-rock was not only an achieved reality by the summer of 1967 (or earlier still, with "Strawberry Fields Forever" and 1966's "Tomorrow Never Knows") but already beginning to be conceptualised.