The sharing table seems like a nice idea. I could see where a lot of people would just be happy to swap like for like, too, especially those who do scrappy quilts. You might have to make a rule about minimum size of the pieces and whether or not they should be pre-washed.
I also can relate to the problem of clannish behavior, which I noticed in the very large guild I used to belong to before our move. My theory is that a lot of people who do a craft like quilting, where you can spend hours and hours by yourself perfectly content, are by nature more introverted. Making new friends costs a bit of an effort, and it's often just easier to hang out with the closest buddies. As a new person I remember trying to take an empty seat and being told that was saved for somebody. Another time I sat with people I didn't know and they acted as if I was interrupting when I tried to participate in the conversation by asking them about their quilting. Then most of what they talked about was personal gossip that an outsider could not possibly fit into. Clearly they would have been happier without a new person at "their" table.
If your guild wants to do something about this kind of behavior, you could assign numbers to the seats and after a few minutes require everyone in seat number 1 (or whatever) to switch to a different table, ideally one where they don't know anyone. Continue to do that every meeting. Mention the importance of trying to get to know people other than your bosom buddies, whom you might invite over for coffee and gossip on others days. I should mention that there were also a lot of very nice people in that guild who were much more welcoming.