This isn't the first and it won't be the last, personally rather than have animal dung moved to fertilise a field, I'd say that you designate the field to be fallow for a season, allowing animals to be penned into the field and generally keep themselves amused and depending how many are there the fields fertility improves (guess why) then when the season ends they are moved off somewhere, naturally this strategy requires some wall be put around the farm to keep animals in, such as fortifications, walls or a fence, naturally a pet blocking gate also is needed to keep them from leaving once designated to field duty.
Such a thing would require animal dung to be rated in richness for fertilisation or just how much they do it, more creature tags as a result.
In future fluids are supposed to be moddable if I recall correctly, so properties of just smelling bad or being toxic/source of illness would go with that making it possible. Sort of voiding the need for this stuff to be put in specifically admittedly you just need to work out how to make it.
From an economy challenge it'd be interesting to have to clean up an accumulation of filth as its not like caves always stay clean with a couple hundred dwarves running about in them creating a need to make garbage dumps away from the population (bottomless pits anybody?) something turned on and off from the init obviously and only starts once you hit a certain population. Though in general with the DF concept of time it just doesn't fit, dwarves cleaning themselves is about as far as it can go. Though scrubbing floors, smoothing walls all so your dwarf gets a happy thought and no filth lies around would be satisying possibly.