Would it be possible to move to a hotel adjacent to the airport for your last night in Venice?
For example, the Titian Inn states on their web site that they are located 900 meters from the Marco Polo terminal, and that (paid) shuttle service starts at 05:00 in the morning:
http://titianinn.hotelinvenice.com/main_en.htm?source=
Trip Advisor has some positive feedback on this hotel:
http://www.tripadvisor.com/Hotel_Rev...ce_Veneto.html