yes to the Fizik's (though I don't like their look), also the Tioga Spyder that pops up on here every once in a while is plastic and well liked (by some), Kashimax made plastic BMX saddles that the fixie kids like here in Tokyo, you could get a fabric covered one--no names except Selle San Marco spring to mind off hand but there are several varieties of kevlar/cordura/embroidery type saddles floating around.

i've been vegan for almost twelve years, no leather for the first 6, then integrated used pieces to my life (started with a practical motorcycle jacket--save my own skin), and then down the slippery slope to buying any leather good my heart desires. still on the vegan wagon food-wise though.....