It's likely that weight is probably not the biggest concern if such a washing system were to be implemented, as they would probably design a system to recycle the washing liquid. I think most solvents that are candidates as washing liquids post either additional health and/or fire hazards in space. In addition, and particularly for water-based washing, I think the error tolerance in engineering either (1) additional infrastructure to the existing water-recycling system and/or (2) a part that can confidently remove unwanted chemicals are challenging problems. the latter is probably the bigger concern because tolerance for error there would probably be substantially lower than urine recycling.
Just a guess because I thought it's an interesting comment, and it's quite possible that one can already design (if it doesn't already exist) a washing liquid that is safe for consumption at small quantities.