Becky Jones of Gonzo Guano Gear has replied to my email regarding the weights of their caving harnesses. She specifies 380g for the GGG Caver Seat Harness and 350g for the GGG Ultralight Seat Harness. Therefore:
Gonzo Guano Gear's Caver and Ultralight harnesses are the lightest.
Petzl's Superavanti is somewhat heavier.
PMI's Pit Viper is slightly heavier than the Superavanti.