I don't see why pumice wouldn't work. I have always used perlite because its available and cheap. I haven't had a problem with it floating as I mix it well with the peat and make sure everything is good and wet before planting the pots (or bog). The difference may be that when I grew Sarracenia in pots, I used shallow water pots and now, they are in a bog. The water level is always just below the crowns of the plants.
