Is it possible to lean over the edge and check that the balconies below are unoccupied before cleaning? If so, perhaps just an admonishment to please check below before sluicing?
I recall a Swiss colleague many years ago saying that apartments there had rules about which day of the week was permitted for airing donnas over the balcony. Perhaps there could be certain allotted hours on a certain day of the week when washing is permitted that might drop water below?