I am piecing together information from this Forum concerning traveling by bus from Cancun to Corozal, and by air to San Pedro. The following schedules are for a Friday departure and a Thursday return.
This is what I've come across so far. If anyone can suggest additions or changes, I would be really grateful.
Cancun - Chetumal: Uno Bus departs Cancun at 00:15 (Daily) arriving in Chetumal
4 hours later. Returning from Chetumal to Cancun at 18:30.
Price: 280 Pesos / $28US
http://www.ticketbus.com.mx/ Chetumal - Corozal: Batty Brothers Premier departs at 6:30 am (or maybe Novelo?)
Might be better, as one article suggests, to take a taxi rather than a bus (~$25US), in that, if I were to take a bus, I would still need to get from the Corozal Bus Station to the Corozal Airstrip.
Price: $3.00 Takes about 1 hour.
http://www.belizecentral.net/bus_schedule/schedule.html Corozal - San Pedro: Maya Air departs at 7:30, 9:30, 10:30, 13:30, 15:30
Tropic Air departs at 7:30, 9:30, 11:30, 15:30, 17:30
Returning Flights: Tropic Air: 7:00, 9:00, 11:00, 15:00, 17:00
Maya Island Air: 7:00, 10:30, 13:30, 16:30.
Price: $70-(RT).
//www.ambergriscaye.com/tropicair/
//www.ambergriscaye.com/islandair/
It appears as if it is quite possible to leave Cancun just after midnight and arrive in San Pedro around 8:00 am, just in time for morning coffee, theoretically.
AmbergrisCaye.com 
