Watch the Triads 1 online guitar lesson by John Stowell from Modern Chord Melody
As you get into applications of substitutions, you begin to realize that every chord, scale and arpeggio has multiple uses. This all feels overwhelming and intimidating until you focus on the individual components of the harmony; music can be approached as a series of smaller challenges and problems to be solved. Absorb and memorize manageable chunks of information and proceed at a realistic pace, sometimes in a shared practice environment with friends. Discussing concepts and playing new ideas with your peers reinforces and clarifies things intellectually and creatively.
Some additional applications of triads could be used in "Haiku", and also have broader implications. There are a number of major chords in this tune. I will use 7 major triads over a given major chord; some of them are diatonic, and others generate considerable embellishment/tension. The trick with substitutions initially is to develop the technical facility to combine fingerings. Over time, two different sounds become a composite. As you move slowly and evenly between the original scale/arpeggio and the substitution, melodies and a general concept will emerge. I also suggest visualizing the intervals to help internalize the sounds that they create.
I will use the following major triads over a major chord: I,flat II,II,III,IV,V,VII. In the key of C, this would translate to using C, D flat,D,E,F,F and B major triads over C major. C, F and G are all diatonic, but playing the triads will generate some different combinations of intervals and sounds other than C major scales and arpeggios. D and E major triads outline C lydian and C augmented major, respectively. A D flat major triad generates considerable tension over C major; I think of it as an implied V altered(G7 alt.), moving away from and back to the C major. A B major triad implies a C diminished 7, which can also resolve back to C major with a slightly different set of tensions.