I think limiting the scope of the filth idea to feces is contrary to the kinds of features Dwarf Fortress is renowned for. Like Bricks I feel that this is the kind of thing that will be easily added through a mod someday. Asking for Dwarves to specifically defecate is like requesting a copper sword before having items. What we are actually looking for is a system that can accommodate some of the game play features we want to do. Example:
-A living unit can emit a splatter contaminant of any material onto a nearby square
-A living unit can emit an amount of any liquid material onto an adjacent square
-The rate and amount of these emissions can be controlled
-This emission can be trigger by any number of unit states (starving, sleeping, just ate, just drank, lost a loved one)
-Units may have zone designated where this emission is preferred to occur and the will task to it.
-The 'emit material' task should be able to interact with built objects or workshops
-The emission can be toggle or triggered on adventure mode characters
-An option for a custom unit need (ex: tiredness, bladder strength, intoxication levels, arousal)
This framework would support the request in question and the crazy part is it seems that Toadys already started on some of these or even completed them. Some could just be outside the scope of the game entirely (haha). If it's flexible enough for others uses and general DF craziness (it's not a bug it's a feature!) then it's very easy to consider spending development time on it and becomes less to do with shit and more about deeper interactions and new possibilities.
Come at the idea from the perspective of story telling as well, our stories about how we felt and what happened are usually greater then the forts they are about. Imagine the story of a drunken human, who wanders outside his house at night looking for a place to relieve himself. He finds a peaceful looking pond and decides it would be a good place to urinate. While doing just that he invokes the wrath of a water spirit living in the pond, who chases after him spraying water in every direction in anger.
There is also the awesomeness factor to consider but I think sewage flood traps, elephants drinking and spraying water about with their trunk, and magma men dripping with molten rock deliver.