In two years, I had some time to think about the "register parking" system that was introduced in this post : http://news.yasep.org/post/2011/11/08/Register-Parking

Now, I have understood the shortcomings and have found a slightly simpler system : the "parking address" is the same for everybody, it's -1.

Read the new description in the updated documentation at http://yasep.org/#!doc/reg-mem#parking