Sorry, I just noticed your thread. I don't know if you are still looking for filter ideas, but I'll send one your way.
I love having a gravity filter system at base camp. I fill it up and have clean water 'on tap'. These systems are simple, robust, customizable, and easy to clean (I backflush mine by turning the filter around so the water flows backwards).

There is a great video by Smokeeater908 on YouTube that shows how he made his.
http://www.youtube.com/watch?v=KDN1aXaTLKA

I did mine pretty similarly, I put the filter further down the hose so that I had plenty of head pressure pushing the water through, and I added a bit of hose (~1.5") inside the bag with a sponge to act as a pre-filter. I also installed some QD fittings into the bag, the end of the tube, and my hydration tube... this lets me run it straight out into a cup or plug it into the hydration bladder for no-hassle refills.
http://www.colder.com/Tabid/72/MaterialID/1/cID/5/Products.aspx

The two filters I would look at would be the Sawyer Mini and the Rapid Pure Scout. Rapid Pure is kind of confusing as far as what you are getting... you would need the filter and the clear plastic housing only.
http://sawyer.com/products/sawyer-mini-filter/
http://www.gofastandlight.com/Best-Hydra...tinfo/W-R-MINI/

This system is pretty inexpensive to make and lets you determine how large a supply you want available.
I can still use the filter as a 'drink-through' when I'm going ultra-light too.

I hope this helps somewhat.