doesn't the 2 degrees only apply to beer in an open container? (5% beer should freeze at about 2,5 degrees to be more precise) Beer in a bottle is under slight pressure so the freezing temp should be lower, shouldn't it?
I´m not a physicist but if you look at a phase diagram of water
you are correct in stating that increasing the pressure results in lowering the freezing point. However, I believe this is negligible in the realm of beer. The brewery pressure is set at around 0.8 Bar (gage pressure and for a standard Lager beer at 3C) and is thus fairly comparable with the data for an open system. Again the best way to do this is empirically, maybe I´ll get around to it sometime!