In general, whether your grain of choice is wheat, rice, barley, maize, or quinoa, you'll need to cook it. And you won't necessarily need pottery to cook it either. Lots of native american cultures cooked in baskets, by transferring hot stones to the baskets which were full of liquid. This amazingly didn't burn the basket.
Chances are this was how our distant ancestors cooked their grains. While pottery surfaced in Japan about 9000 years ago, it took quite a while to disperse.
So a similar thing could obviously be done with beer, and the beer could easily be stored in watertight baskets, and wouldn't exactly leave a whole lot of evidence for the archaeologists.
And before you ask about watertight baskets, I have an Eyak basket made of seal gut which is quite watertight. Anyway, you could always seal the basket with animal fat.