I finally removed the "transitory" feature of being able to allow kin onto a plot, and replaced it with a more complete way of managing permissions. With the new system, you can choose, for each group of memorized people, what permissions they should be allowed on the plot in question. The available permissions are based on the type of crime clue an action would leave; you can individually allow Trespassing, Theft and Vandalism actions to be done on the plot.
Of course, there probably won't be much meaning to allowing Theft or Vandalism without allowing Trespassing, but I'll leave that to you to decide.
Previously shared plots will allow the green group (in which all of your kin were placed when the new kin system was implemented) all permissions, and no permissions to the other groups.
This system should be quite usable, I think. Enjoy!