There was a point this weekend when it looked as if a large number of my current FC mates and I would be starting our own FC, so we went out and I bought a medium plot (I had the gil to spare). While it couldn't be converted into an FC house, our method (had we needed it) would have been to grind the FC up to 6, and then when we had the gil saved to purchase the plot, I would have bought a small, throwaway 1 mil house in the FC's name, relinquished my medium, and relocated the FC house to the medium.

It's double-purchasing, but depending on the server you're on, that might be worth it rather than trying to camp it and pray for the best.