I hesitate to suggest this, since I'd prefer my dwarves to use nuclear fission for their digestive needs until (if...) a more detailed system is properly added to the game, but I think you can hack together a simulated waste and sewage system with a bit of imagination-stretching. The basic principle is stolen from some plans for an automatic quantum dumping machine, wherein the operating chamber is replaced with a stockpile. There are several points where I'm not sure if this works due to my never having had nor wanted to build this.
What you'd do is mod in a 1x1 workshop that either consumes nothing but dwarven time and labor or a single unit of food. This workshop would create a unit of "waste," which would sit in the shop and be used for nothing. It could rapidly or immediately begin to spew miasma.
The important part is how to automate the disposal system. You could create a 1x1 room, sandwiched between a door, a waste catchment pit and a pump to simulate a flush toilet. There would need to be a long corridor infront of the room, in which lay a carefully designed mechanism to ensure that a dwarf would only trigger the automatic flush system when it was walking AWAY from the water closet. This system would cause water to flow from the pump next to the toilet into the catchment pit. As long as the water flowing over the workshop is not 7/7 deep, it should cause object-moving flow to occur, pushing the waste into the pit.
water╔═╗
┌-- ║·║ <-- Chute to clean water cistern
v ║%║ ^ pump
╔════╝%║ | direction
║·%%‼╥·║ <-- Chute to waste catchment
╚═══╦┼═╩═══════════════════════
->║++++++++>+^╞═══════════╡^+
pump╚══════════════════════════
direction 4 3 2 1
‼ = vertical bars prevent anything from following the dwarf in and standing here, as a stand-in for whatever it takes to allow the water to spread so it's not 7/7 deep and thus has flow
╥ = porcelain throne
The player would queue up a "Disposal" job in the water closet. A dwarf would feel the need, and subsequently walk over the entry pressure plate (1), causing the bridge (2) to open and drop the dwarf one zee-level down, in a short tunnel leading back in to the main tunnel by way of the stairs (4) placed
beyond the second pressure plate (3) to ensure the flushing mechanism would not be triggered while the dwarf was present.
The dwarf would use the modded-in reaction to generate a particle of waste in the water closet's 1x1 workshop building (╥), then leave. On the way back, the dwarf would trip the flushing mechanism (3) and safely walk over the bridge (2), since the bridge would not be connected to the flushing mechanism. Tripping the bridge trigger (1) on the way back is not an issue since there should only ever be one dwarf in the loo at any given time.
When the exiting dwarf flushes this device by stepping on (3), a screw pump (left of the john) activates and sends a
small amount of water, somehow properly measured so as to ensure it does not exceed 6/7 water units and thus has the property of flow. This would push the miasmatic filth into the catchment pit. The water in the pit would then be drained somehow, perhaps by an additional pump.
Reasons this may not work well:
- I'm not sure if items generated in a workshop will be subject to the push effect of water flow. This one's a dealbreaker. If items can't be water-pushed out of the workshop they were generated in, the only alternative is to have a dwarven waste shoveler... the least impressive solution. Or you could persuade some goblin captives to do this for you.
- There may be numerous circumstances where creatures could follow the dwarf in and stay there (make the entry/exit corridor longer to increase the likelihood they'd follow the dwarf out in time).
- Getting water flow to behave predictably may be troublesome. If this is a problem, it may be remedied with sufficient space and redundant flush pumps hooked up to the pipe focusing the same "workshop," so you can get probability on your side. Eventually one of the flushes would flow the waste item into the catchment pit.
- Dwarves would need to be told to "relieve" themselves via job queueing. This could be set to repeat and made to require a unit of food, though this would be a pointless loss of (trivially produced) food items, and one dwarf would end up being the only one using the restroom for an entire year.
Of course, even if none of the above is too much of a problem, the dwarven flush toilet would still be about as useful as an extraordinarily smelly, ugly and complicated temple to Armok, with none of the screaming evil priests, chained ravenous horrors and pentagrams drawn in blood.
... Please.