I had a serious think about this, I haven't flushed the thing once since thegame came out, and we don't have anybody sucking up your well dry either. Maybe a mod can do that? pay another neighbour to suck it up?
I'm not even sure this needs to be a mod. I feel like this sort of thing should be included in the base game by the time the game comes out. Considering the ****-tonne (see what I did there) of things that are being added each update, I wouldn't be surprised if this was an eventual feature.