I'm working on a setup for garbage disposal. It doesnt need to be flowing all the time, but yes it's hard to pull off. As soon as you get a bit above the 3/7 you have job cancellations all the time (from the dwarves that carry the item to the flowing area).
But I have a new plan that might as well be the breakthrough.
First (that's still the not-perfectly-working version) I just tried to find a balance between pumps and water inlet. Ran fine for a few months, then it got flooded and I had to shut down the inlet for a few days (estimated). Too much micromanagment and too many job cancellations.
Second step was to connect my water inlet with my patented animal alternator. Basically its just an animal chained up next to some pressure plates. The size of the room basically determines how often that animal will step on each plate. In that case you need an 1x2 room, 1 plate, 1 chain and you can say the door is open 50% of the time. Worked better than before, still job cancellations and micromanagment.
Now, and I havent built it yet, but I think it is the final step to make it work, I have this plan:
1x3 room as alternator. 2 pressure plates, chain in the middle.
Flowing setup looks like the "old school irrigation" on the wiki: you calculate how big of a reservoir you need to fill your let-the-items-flow-area with 3/7 water. then you build two floodgates, one between water source and reservoir, 1 between reservoir and the let-the-stuff-flow-area.
The animal has 3 tiles to stand on: middle tile, which closes both floodgates.
and two pressure plates, each of which opens one of the floodgates.
you will have a WEAK flow and sometimes the room will be full with items before the dwarves can bring new ones.... but there should be ZERO micromanagment and job cancellations.