I am currently looking into non-functional but still directed harmonic music.

Frank Zappa might be a inspiration (tall order).

A technique I learned is chromatic planing, which uses coupling of one or more chord structures to a top melodic line.

For the classically trained musician the parallel motion might be a no go, but I personally like it a lot.

Opusmodus is perfectly suited to do this planing technique with gen-chord3.

Very brief snippet, which was arranged mostly with music generated from code below.

(setf melody-interval (interval-series '(1 5 7 5 1) '(1 2 7)))
(setf melody-pitch (ambitus '(c4 g5)
                            (interval-map 'd4 melody-interval)))
(setf melody-length (gen-tuplet 1 1 'n 'n 'w (mclength melody-pitch)))
(setf melody-omn (make-omn :pitch melody-pitch
                            :length melody-length))


(setf chord-interval '((-3 -7 -10 -12)   ;; m7
                       (-4 -7 -11 -12))) ;; maj7
(setf chord-omn (gen-chord3 melody-omn chord-interval :cycle t))
(setf rhodes-omn (closest-path chord-omn)) ; :type '(1 2 3 2)))
(setf bass-omn (omn-replace :articulation 'stacc
                            (pitch-transpose 0 (dechord chord-omn))))
(setf synth-omn (pitch-transpose 12 (dechord chord-omn :pitch 4)))
(setf marimba-omn (pitch-transpose 0 (dechord chord-omn :pitch 3)))

(ps 'gm
    :tempo 112
    :pg (list (length-weight rhodes-omn :weight '(2 1)))
    :bass (list (length-weight bass-omn :weight '(2 1)))
    :fl (list (length-weight synth-omn :weight '(2 1)))
    :ob (list (length-weight marimba-omn :weight '(2 1))))

(write-midi)

It was indeed the idea of gen-chord3. Originally I wanted it to be able to control the harmony below a melodic line.