I have seen this done with tents set up, but am not sure just a pack sitting there would be enough to convince another party to move on.  I've respected that a tent (empty) was set up and have moved on, in spite of not seeing anyone who appeared to belong to the tent.
I've asked if a party (who looked like they were leaving) was actually leaving and if I could hang around just off campsite until they were gone and take the site over.
Once a lady friend and I hit a campsite late in the afternoon on one side of a point while a group of men got on the same site from the other side of the point.  We met at the top in the middle, we tried to claim the site but they wouldn't relenquish it without an argument and I wasn't anxious enough to get beligerent so we ladies moved on... >:(