Personally, I would not want to spend the entire meal watching the clock to make sure I get out in time.
As the PP said, the Biergarten is a buffet, so you can pace the meal yourself. They general seat fairly on-time, but I've had to wait after my ADR time to be seated (I think the max was 30 minutes).
So, overall, I would say it would be possible to get out just as IllumiNations is starting, but you won't have a lot of buffer time and you might have to just find the first available spot to watch IllumiNations.