I thought the TV show I saw had the pine needles on top, but I tried it that way we got
mouthfuls of ash, though very tasty mouthfuls of ash. I thought I must have gotten it wrong. We didn't use a grill; we used the fire pit. Perhaps on the grill the needles will burn away and fall through by the time the mussels open. Now I'll have to try it again on my Weber grill.
This is a tradition from Les Landes, south of Bordeaux, where pine forests grow next to the ocean.
Here's a different method from Provence by Richard Olney that I haven't tried yet: Open the mussels live (that's why I haven't tried it), leaving them in their half-shell, and arrange on a grill. Sprinkle each with pepper and a few drops of olive oil.
Meanwhile, make a bonfire of dry grapevine prunings (another reason I haven't tried it). After a moment of fierce blaze the flames will die and leave a mound of glowing embers. Spread the embers into a flat bed. Place the rack with the mussels right onto the coals.
"When the mussels contract, releasing liquid that begins to boil almost immediately, combining with the olive oil to form an exquisite little sauce, they are done. Remove the grill from the coals before the mussels' liquid has evaporated. Pour well-chilled Tempier rosé and place a bucket on the ground to receive empty shells"
Richard, if you try this, I'll raise a glass of rosé in your honor.