We had some friends do what you are suggesting - they got an early tender (not first but not last) and got the 9.30am flight from Municipal (there is no security etc at Municipal, you just turn up and take the next flight which are on the half hour). We picked them up at the airstrip and were "out and about" by 10am.
Going back they got the 3pm from San Pedro (all flights are on the hour going back) to get their 4pm tender. So it is possible and you would get about 4 hours of "island time".
Exactly what we did. We ended up on the first tender and easily made it back for the last tender at 3:30pm. We got just over 4 hours of snorkeling and beach time in AC.
Go to Youtube and search "mdrobtx" for videos of our excursion to AC from Carnival Legend.
AmbergrisCaye.com 
