As far as I know, it's not possible unless someone leaves the saferoom "nav area" even without opening the saferoom door (which can be done in c2m3, the rollercoaster level) and every Survivor client is programmed to spawn INSIDE saferoom nav areas.
This is what is causing premature round starts in levels like the rollercoaster level:
Every saferoom has a nav area which is labeled as "PLAYER_START" or "CHECKPOINT" and are color coded as such. Every nav area labeled as a blueish/cyan color denotes a "CHECKPOINT" nav area which means that it's a saferoom and the round hasn't started yet until someone leaves that spot to an adjoining non-"CHECKPOINT" nav area.
However, in this screenshot, you'll notice the nav area I have my crosshair on is NOT blue:
This means once someone steps into this nav area, they will start the round. These nav meshes cannot be fixed (actually they can, but I believe will require ALL connecting clients to redownload the nav file, overwriting the official one which could and most likely will cause connection issues). The only kind of workaround is what I mentioned earlier, but ultimately won't prevent rounds from starting if someone steps in those nav areas.