My original idea was discussed here.
I had the idea to connect a three-way connector close to the hydration bladder to connect a water filter's output hose to for a seamless way to pump water into the bladder.
I am aware campmor sells this, which is useful but for me to fill up, I don't have to disconnect anything. I just need to pull my filter out, drop it's input line into the water and pump. This is the perfect solution for myself. I hope others find this setup useful.
I found the part I needed for $2 at an auto parts store (Strauss.) It is called a 1/4" Vacuum Tee, these are meant for vacuum systems in cars. Some persons recommended finding the part in a pet store as similar hosing systems are used with fish filters, but the only fittings I saw we're 1/8" Tees, way too small.
A 3/16" Tee will work as well, but this will restrict water flow significantly and the fitting isn't as tight (pressure in the hose will pop it right off.)
Installation was very simple, I just cut the tube and put the fitting on.
My Hardware is a 3.0L Platybus Bag with Hoser Cap, I'm using a CamelBack's Hose (with it's on/off valve and bite valve.)
The filter is a Katadyn Hiker Microfilter Pump.
I bought a Vacuum Tee Assortment Pack, this website is selling the same thing and has pictures.
I might add an on-off valve on the Output Line near the filter. There's very little danger of me backwashing anything into it and I haven't noticed sucking anything forcefully out of the filter, but I can't imagine that being great for it. With a 3.0L bag, I don't anticipate ever running out of water to where I destroy something by sucking too hard, but I have the feeling I should have the output hose off when I'm drinking.