i dont see how a bov could work both ways. inside of there is going to be a spring and a valve. if you installed it one way, the spring would push against the boost to try to keep it shut, but the amount of boost it would hold would depend on the springs tension. after a certain psi, they would leak boost.

if you install it the other way, then the boost would help seal the valve and it could hold any amount of boost. the spring would be there to help assist it keeping it closed under minor changes in vac or at idle.

i would install it the 2nd way myself, unless you know exactly how much boost that bov was designed to hold.