Can I use sewer cleanout as washing machine water outlet
Not a 100% definite answer (I am not a plumber or a code expert), but I would be concerned about a trap. Or rather, a lack thereof. A plumbing fixture - sink, shower, bathtub, dishwasher, laundry drain, etc. normally has a trap that fills with water and keeps sewer gas from getting into the building. A cleanout, by its nature, will likely have a straight shot down the drain pipe - i.e., you would not want to have a trap in a cleanout because it would make it much harder to clean out.