If you're only going to take only a couple hits (since 1 will get you high for the next 4 hours), don't worry about smell or ventilation or anything. A couple hits isn't any more likely to set off an alarm than a steamy bowl of soup (and in the extremely unlikely case that the alarm does go off, just get out of there). You can probably dash into any bathroom stall, take a couple hits and leave in the span of 2-3 minutes. Nobody will ever have to know anything. But yeah, if you're going to go to the trouble of using the ventilation system, it would probably work.