I don't know of a direct WDW to BG bus. But there is a Sea World bus that first picks up at Sea World, then several stops along 192 before heading out to BG. It's ten bucks per person.
We went in the off season and only had to leave the park like an hour before closing. It was also really nice because on the ride home we hit a really bad thunderstorm, and I wasn't the one driving thru it.