The rule is the same at every ground, you can't sit in a box and been seen having a drink during the game.Aye, the riff raff do find their way into them.
So if someone is sat at the table, sipping a pint, blinds are closed at all times.
Genuinely interested to know.
However, what happens at most is that they tell you to use a coffee cup and simply pour your wine or G&T into it and then anyone looking in is none the wiser.