I've been playing around with a gravity feed filter system. I recently tested how well it could be used to fill my source bladder in my backpack, and it worked surprisingly well. My setup is a Sil-nilon water proof stuff sack with a hose fitting on the bottom, and about 3' of hose, and then the filter, and a connection to hook to the quick connect on the bladder hose. To test how well gravity would fill the bladder, I started by filling the bladder from the faucet. Then I packed the backpack completely full, around the full bladder. When the backpack was as packed as I could get it, I emptied the bladder. I filled the stuff sack with 2 liters of water and hung it about 6-7 feet off the ground, with the backpack sitting on the ground, and hooked together the hoses. Finally, I opened up the system and timed how fast it ran. The two liters of water in the stuff sack emptied into the bladder bag in about 20 minutes. This is with a cheap crappy filter I had sitting around, which doesn't flow very well. In fact, it takes about 15 to 18 minutes to drain 2 liters out onto the ground, so letting it flow into the bag didn't really slow it down that much. I think that with a better filter, I'll have a real light and easy to use setup that works about as fast as a pump. Even if it is a little slower, the fact that I can set it up and go do other things still gives it the advantage.
If the intention is to use a steri-pen, would it work to use a water proof stuff sack instead of a water bottle? the stuff sack would probably be lighter, and it could then be used for gravity feed.