I know this isn't an ideal world, but why are we still putting crew under this sort of pressure? If you are a new F/O and flying with Capt. Bloggs who just happens to be no.1 on the seniority list this is an almost impossible situation. You are damned if you do and damned if you don't.
With this situation in mind, why does BALPA keep insisting on peer group monitoring? In this scenario, there is no peer group equality.
I for one would have no problem 'blowing into the bag' before duty, since I never have a drink the night before flying. It is just one of the things we have to accept as crew. Is there anything wrong with having a beer 12 hours before going flying? No of course not, but personally this career means more to me than a single drink, If you want to be sociable with your colleagues down route you can always have a soft drink so as not to run the risk.
Mind you at the end of a long hard day, a nice cold beer can be a god-send.