Being a noob myself I would imagine that I'm off target here referencing the crashcart, but this is what I think of.

An external micron filter HOB to quickly clean/polish water.
Ozone reactor to crank out some serious nutrients in a in-need tank.
A large phosban reactor that can handle a larger amount of water volumn.

This would be an effort to quickly reduce the problems in the water when a WC can only do so much (limited by how much water you carry with you).

Additionally, and this is just my new guy's opinion, I would steer away from premade salt water over RODI standby. Here's my reasoning. What do you salt at? 1.022? 1.025? It only takes a few moments to mix, indeed it seems bringing the water to the proper temperature is what takes more time.

In addition, how long can you keep water sitting in a container before those elements that are impossible to completely keep at bay are not allowed to find a home in these buckets of water?

Just an idea, feel free to shoot down or destroy at will, but how about this. Take all volunteers that are willing to toe the line and keep on hand at all times a five gallon bucket of RODI. A pledge really to always be ready to contribute a bucket or two, therefore to always have one on hand. This will allow us to constantly First in First out these buckets to keep them fresh.

If you had a group of 20 members willing to standby chances are you will at any time have access to at least five members who can respond. So perhaps with fifty members of the Bucket Brigade (suddenly I'm hungry) we would have twenty buckets in a time of crises. Five gallons each (ten for some) and you have 100 gallons of RODI at the ready.

A simple phone roster would be enacted rather than a call to arms over the Internet. I thnk in the civlian sector it's called a phone plant or phone tree.

Anyway, it's early and I'm just thinking out loud. Sorry about being so wordy.