While I respect your knowledge of the industry, I'll just point out the following:
1) DME info DOES make it to the resort level on a routine basis. Every day, resort cast members are printing personalized departure notices and delivering them on guests' doors. On some level, each resort knows exactly when guests are leaving via DME.
2) We know housekeepers are given lists of rooms that need to be cleaned. Is it really so far-fetched to believe that in 15 years of DME, someone took the time to funnel DME departure times onto those reports? Across 30K hotel rooms and villas with a 90% occupancy rate, isn't it worth SOME programming investment to make that happen?
3) Instances of housekeepers knocking on doors looking for vacated rooms are still rather rare. In my own case, maybe 5% of my stays. If most / all housekeepers arrive at 8am and their only means of finding rooms to clean is knocking on doors, I suspect these boards would have been overrun with daily complaints by now.
4) So why do the door knocks occur? Simply because not everyone uses DME. Resorts don't have departure information on every guest. If all of the known departures are taken care of--or all in a certain individual's assigned section--time to go looking for new rooms to clean.
5) If Disney truly cannot do something as rudimentary as pass along known departure times for a large block of their guests, is there ANY reason to believe they would be equipped to handle people self reporting their departures? Whether a guest calls (call center?), stops by the front desk or makes some notation on a Disney app that they are leaving, do we really believe that information will be immediately disseminated to staff scattered throughout the building?