Quite a nice touch indeed, agreed. I think this idea (might actually have been you lol) has been brought up briefly a couple times on disjointed threads, but the idea of recycling the waste from the Dump Zones is something I'd really like to see.
When the hygiene system is more developed than "Carried Dirt: Sand x2" lol or more basic needs are added to colonist so they can excrete rather than just store it up somewhere in their digestive tract with a cork, I think looking into this would be really important to overall gameplay.
Sewage and animal waste being used as fertiliser is a tradition that's been around since the earliest civilizations and if you were starving on a not-very-fertile Rimworld you'd want to get the best harvest possible.