Actually, I've just found and read this now (here), and it's much closer to what I was hoping for / expecting from The Great God Pan than the novel was itself. Although its Pan is largely malevolent, that's because we're seeing him from the perspective of Sylvia, who isn't really attuned to the countryside generally, voices disbelief in him specifically and also commits the sacrilege of casting aside his offering. The implication is that his golden laughter, beautiful brown face and piping music would be bestowed entirely benevolently on true devotees - e.g. Mortimer.