I have a platypus filter and it also cautions against freezing. I wonder though if the concern is the filter freezing with some small amount of water still in it, given that the water would expand inside the filter. I.B. says they let theirs thoroughly dry before freezing, so maybe it's ok that way? I'm just speculating here though.
I have also used a Steripen and it's great, but it obviously doesn't filter for particulate matter, so you either need a clean fast running source to collect from or a tap. In a pinch I suppose you could strain some muddy water through some cloth to clean it a bit before hitting it with the Steripen.
Edit: I see djb already addressed my point about freezing with water in the filter above. Must remember to read before posting.