Are there requirements from the coach or an expectation that they wouldn't drink at all on the day of a game?
The players I know stick to a pretty professional schedule, even for ABL level. Pre-game routine, resting at certain times, eating at certain times. You'd be hard-pressed to get them to stray much from that.
(Can anyone remember that time Darren Ng copped some criticism for having been at an event or family BBQ in the lead-up to a game? I can faintly recall something like that.)
Perhaps less so if they're unpaid, but even at ABL level, I'd say that there is an expectation that you are picked to perform and set yourself up for your best chance to be at your peak on game night. Clubs spend on players, coaches, venues and they all take the competition seriously - with that comes the expectation I mentioned.
If the coach wanted to send a clear message, I don't think some sort of suspension is out of the question.