Sounds like a good plan. Here are a few suggestions:
There is a weight penalty with the Big Zips over a regular Platypus or Sawyer Bag. I think the Sawyer bladder is lightest, but not very durable. A regular bladder is a only a tad heavier, but much stronger. You can put holes in either a regular Platypus or a Sawyer bag in the corners (not in a spot that would cause it to leak) and then attach some cord. Using some more cord, you have a dirty bag that you can easily attach to a tree, rock, or anything tall.
There are three ways (that I know of) to create a drip system. One is to use connector pieces (available at REI or other places). This involves a small bit of tubing. The drawback to this approach is that the tubing will fall off if you pull on it. In other words, you can't just connect the pieces and hang the whole thing by the dirty bag. The other two approaches solve this problem. One of those methods is to glue two caps together and then drill a hole in between them. The third is to buy a Tornado Tube and use it to connect the pieces. You will have to modify the Tornado Tube a bit. One part of the Tornado Tube needs to be sawed off (a hacksaw works great). This will become obvious once you buy the Tube and try to attach the pieces. It is also a good idea to make the hole in the tube bigger (with some additional drilling).
I've heard people create a pre-filter, but I forget the details. If you search for Sawyer Squeeze tips you might be able to find it (or someone will remember the details). I don't think the filter will completely clog, but it may get really slow until you back flush it especially if you filter a lot of silty water. I haven't worried about it, but I probably have to back flush more often than if I used a pre-filter.