You may have misunderstood me. If you have to drive when it's zero degrees out you may want to go up one O-ring on the dip stick. The engine is still sealed just gives it a place to release high pressure in case of the catastrophic OS failure. Like a donut tire when you get a flat, it's temporary so you can continue to go. You certainly don't want to use it all the time.
Remember the newer X5 has pressure relief on the valve cover, same concept.
Although I like the suggestion about a pop off valve on the oil cap. A second oil cap could be used, fitted with a valve and put on only in extremely cold weather.
Have you guys seen the service bulletin from BMW Service Engineering about the heat wrap for the vent system which connects to your battery? Here it is but without the pictures.
SI B11 01 04
Engine
March 2004
Service Engineering
SUBJECT
Heated Wrap for Crankcase Vent System
MODEL
E53 X5 with M62 engine from start of production through 9/2003.
SITUATION
During periods of driving in extreme cold weather conditions, certain X5 vehicles may experience misfiring, blue smoke from the exhaust, high oil consumption/oil loss or engine damage.
CAUSE
Condensation in the crankcase vent system could freeze, causing higher than normal crankcase pressures. In extreme cases, this could cause engine oil to be introduced into the intake manifold and then into the combustion chambers. Should sufficient oil quantities enter the cylinders, engine damage could result.
CORRECTION
Install a heated wrap kit around the crankcase vent hoses.