Thanks for your suggestions. Some of them were made in earlier posts on this thread, and I responded. I sort of got wrapped up in this, so spent a lot of time browsing, and called McMaster-Carr for details, among others. Googled in many categories, like aquarium pumps for example. Everything was much too heavy.
The lightest pumps out there seem to be air pumps for bikes; but I don't know how to, or even if you can, convert an air to a water pump. Also, am not savvy enough to find syringes and check valves that are durable for long term use, and figure out how to make them work for this application.
Don't have time to tinker with this now, but maybe next winter. In the meantime, will talk to folks in healthcare professions to get an idea of what might be out there. The lighter Radio Control fuel pumps are crank types, the gears tend to strip, and are not what I'm looking for.
You would think a durable small piston liquid pump would not be so elusive, but it seems so. It may be because those wishing to move water and other liquids by hand find a larger pump more suitable. I've no doubt that with a 2 oz Sawyer filter, a pump filter with a total weight around 5-6 ounces is totally feasible, so will keep looking.
Will post, of course, if successful. It took me two years to find out that kite fittings were totally worthless, and another year to figure out how to modify plumbing fittings from McMaster-Carr and other places to make pack frames; so I've learned to be patient with this sort of thing. But it often takes time.