I’ve only played 2 or 3 productions of Amahl so I’m not positive about what’s the usual, but I’ve played a gliss. Sort of a fast flurry of left, right, left, right, left right of descending lines. The pedals are D#, C-natural, B#, E-flat, F#, G-natural, and A-flat. In fact, in one production, I played the glisses at 34 and then rapped my knuckles on the soundboard four times for the Magi’s knocks at the door.
…I just checked the CD I have and the harpist plays glisses. This was the original NBC cast recording with Menotti directing and Thomas Schippers conducting, so I figure if that harpist glissed, I would as well!