Watch the Soul Mates online guitar lesson by Matthieu Brandt from Guitar Lab: Triads and Hendrixian Double Stops
In a backup part containing just triads you don't have to play the complete triad all the time. You could limit yourself to playing only two notes of the triad.
Seeing the triad shape, which is basically the heart of the chord in the background, makes it easier to find interval that will sound good. Once you know the triad in the progression, the intervals are right beneath your finger tips.
This next soulful example uses this technique of splitting up triads to create a backup part that moves all over the neck.