I don't think there is a harpist on TC who can verify or contradict what Bill Marx, Harpo's son, reports in his book Son of Harpo Speaks!
"It wasn't until I was sixteen years old, and became his arranger and conductor, that I fully understood the complexities inherent to the harp. (Harpo) explained it to me this way. It's got forty-seven strings, representing all the notes on the harp, seven pedals at the base of the frame, each one controlling the string for each note in the scale, and three pedal positions that create all the necessary sharps, naturals and flats by lengthening or shortening the string in order to produce the note desired. Now get this. There is only one possible way to play any single intended note correctly, let alone a cluster of them at the same time. To begin with, you have to pluck the right string for the right note you want. While you are doing that, you ave to fing the right pedal that corresponds to the string for that specific note, and then pray that you chose the correct foot, so that it can push that particular pedal into the appropriate position to get the note you hoped to play in the first place. All this goes on at the same time, in a split second.
"When it's presented to you this way as Dad presented it to me, it really does sound pretty "iffy," doesn't it? The odds are stacked against your ever playing the right note. He said that if you multiply the forty-seven strings times the seven pedals times the three pedal positions, you will find for every single note that you want to play correctly, there are nine hundred and eighty-six ways to screw it up! And that's just per one note! I figured right then and there that it would always be a much safer bet to write for the harp than to play it. Besides all that, I've come to learn that most harpists spend fifty percent of their time tuning the harp and the remaining fifty percent of their time playing out of tune."
Bill reports that Harpo was very good friend with two of the most distinguished concert harpists in the world, Marcel Grandjay and Mildred Dilling, and that Harpo was the one who taught these harpists how to get vibrato out of a harp string (harp strings produce a pure non-vibrating tone when struck).
Bill must not be referring to bisbigliando tremolo effect, as he reports that neither harpist had ever seen Harpo's particular technique in harp literature, and both incorporated the technique into their respective recital repertoires.