Though in the morning it is very hard to catch a bus back to a resort, because many times the buses themselves don't know where they're going until they're leaving the area (not many people are leaving the MK or other parks at that time, KWIM?)
The buses receive their next dispatch prior to arriving at MK and will stop at the appropriate bus stop, if someone is there.
In addition a spotter, with a radio, walks the bus stops asking people where they are going and making sure they are in the correct location. When a guest is returning to a resort or going "upstream" the spotter will radio the dispatcher to notify the next driver, scheduled to go to that resort, that there is an "upstream" that they need to pick up.
In addition to this, many drivers will call in on their radios, as they pass a bus stop that has guests, standing in the queue.
This is also done at Epcot and to some extent at AK, while HS visually dispatches buses as needed.
So getting a bus to a resort, early mornings, from MK should not be too much of a problem, as long as you are waiting right at the bus stop, for the resort you want to go to.