I think all the harlequin scenes are so unspeakably dreadful in the original that I can't imagine a well-made TV version, unless they re-wrote all the de Momerie chapters

Nah - all you have to do is acknowledge that every single person involved in them (including Peter) is high on something. Film it in black and white, make it explicitly hallucinogenic, wibbly under the moon stuff. Show the fountain next day - not high, with deep, safe basin - compared to the olympic diving board height you've previously illustrated it as, play up the glamour vs. tawdry reality stuff. Make the cars impossibly fast, the clothes impossibly tight, Peter thinking of himself as a comic book hero, and the whole thing a devestating critique of market capitalism and it could be fantastic ;-)

Oh, and shoe-horn Harriet in as much as possible!