I like the theory of vacum/negative pressure on the crankcase,but with all the aftermarket breather systems,the crankcase will only vent naturally.a cosy engine builder told me that a problem he has seen before,is the head filling up with to much oil,and or,not draining away quick enough,and overwhelming the stem seals.
When the head drain becomes overfilled with oil it causes a slight airlock(a bit like when you turn a water bottle upside down,the water glugs out,If you made a hole in the top of the bottle,the water will flow quickly out.The point i am making,if the head is connected to a breather,which in turn is connected to the crankcase,you will get "0" pressure in the head.Now i'm thinking that you would want to vent the head separatly to air,and have a vacum in the crankcase to pull the oil through