This is all speculation based on the fact that the parking charges will be added to your hotel bill, and that I am guessing that hotel guests would be expected to register their vehicle when they arrive.
First of all, if you drive to a hotel you aren't staying at, you won't/can't be charged the daily parking fee, but you will still be subject to the four hour limitation, and there is already a system in place for that.
Second of all, Disney would need to implement some type of control to exit resort parking. Scan your MB when you leave, and they will know if your reservation requires to pay for parking or not. If it does not (DVC member, reservation made before 3/18/18, etc.), then fine go ahead and leave. If you are required to pay, and if the exit control confirms you have a car registered to your room, then fine go ahead and leave.
However, if you are are required to pay, and if the exit control confirms you DO NOT have a car registered to your room, then the parking charge is added is to your bill. Guests not staying at the hotel would be issued a ticket upon arrival, and must surrender it when leaving to prove less than four hours have elapsed. Lose your ticket? Then you would be charged the daily on-site fee.
Once again, this all speculation, and the actual implementation could be completely different.