Only tangentially relevant, but for the benefit of anyone with the older single-piece HPV hardshell seat, the trick for reducing (on a good day, eliminating) the incessant creaking is (unintuitively) to clean the seat/frame interface with degreaser/alcohol and then do the QRs up bastard tight. Ideally, do away with the QRs entirely and fit proper bolts.
They fixed this particular problem with the Bodylink seat. Something to do with silicone washers, IIRC.