Normally yes. But from personal experience, I can tell you that behind that fence on race day people are crammed cheek-to-jowl, and there may be literally no room -- not so much as a single square foot -- on which you could place a parcel without laying it on someone's unsuspecting feet. One time my friends were supposed to pick me up at the finish line to drive me home. They were no more than 100 yards away, and we still couldn't get to each other, the press of the crowd was so impenetrable. I ended up hitching a ride with another acquaintance I happened to blunder into instead.
If I were jammed up against the fence like that, I might very well set my backpack down on the other side, where there's plenty of room (and the fence is so low you can still fetch it easily).