Re: Custom properties

I would modify your requirement (interface CustomProperty) as follows:
(*) The class has to provide a public constructor taking a String
argument.
(*) The class does *not* have to provide a fromPropertyString(String)
   method. (hence you don't have to give up immutability)

Obviously the loading from a string gets more clumsy then.
But at least one can tuck away that clumsiness into a utility method:
 public static Object createFromPropertyString(String s) {
   try {
     Constructor ctor = c.getDeclaredConstructor(new
Class[]{String.class};
     return ctor.newInstance(new Object[]{s});
   } catch(Exception e) {
     throw new RuntimeException(e);
   }
 }