Slim,
Your right, that's a definite "Stevie-ism"....
Try clicking the tip of your tongue of the top of your upper palette, 1st without the harp and you'll get a a sort of click-pop sound effect. Then, take a few notes on the chromatic (upper register works best above 5).
When you first hear it it sounds like his giving the harp short staccato type "Kisses" but it's really the technique above.......
The Plucking in his version of 'Alfie' is awesome!
Good Luck............ Rob Paparozzi