I like this. Unfortunately it's not as semantic as I expected:

            X
     +----+XX----------+
     |     XX          |
     |      XX         |
     |       +XXX------+
     |       |   XXXXX
     +-------+       XXX

     If I try to resize
     a rectangle that's
     crossed by a free-form
     curve, it stops being
     a rectangle.

(Now that I think, I could also use this as a feature).