For what it's worth, I got politely turned away at the PostgreSQL devroom doors when I was about 10 minutes late to a session. Perhaps this is not something that got passed along to all volunteers.
Could have been because the room was full. That's AFAIK the only reason why lecture hall doors are closed at FOSDEM. At least Postgres has a room of more or less appropriate size lately. A couple of years ago, they had some of the most interesting talks in one of the smallest rooms. Impossible to get in without queueing up a talk or two in advance.