And if this ever gets added, who wants to bet how long it will take for someone to start drowning sieges in dwarf poo? I say 5 minutes.
This is dwarf fortress. Everything is weaponised at some point. Of course, in this case, it wouldn't be particularly spectacular. If RavingManiac's suggestion were put in place...
Also, I do not support treating nightsoil as a moving liquid due to the resulting increased complexity. Instead, it should be treated as an object, which generates miasma at periodic intervals before eventually decaying into fertilizer, and disappears in water, turning it brown and undrinkable (and since it will not disappear in water that has already been contaminated, flowing water will be neccesary).
(Re-quoted for your convenience)
... then it would be exactly the same as every other drowning system. Well, except for the part where there's an outhouse above it at some point, thereby possibly creating a weakness in your fort for invaders to exploit. Good job.
If a player wishes to be crude or juvenile, then that's their call. It doesn't particularly concern me what some kid in <insert location that isn't my house> is doing on his own computer. Things should not be added to the game just for the eight year olds to chuckle at, but neither should things which are otherwise sound and interesting ideas be excluded solely to deny them their irrelevant little thrills.