[JayDaley]

I too would like an answer to this question.

I've found one way to create an "open shape" which is to draw the path back along itself to the origin so that the shape is actually closed has no internal area. Unfortunately that method cannot create an open shape like the "square bracketed box" (aka "double square brackets") as it only works for a continuous path.

Come on Omni, please explain if these shapes are created with some magic sauce we don't have access to or if there is a trick we could use to create them.

If you need a good reason then think of all the Visio users who switch to OmniGraffle and look in vain for a callouts stencil to rival that of Visio. The reason there isn't one is because we can't create the complex open shapes that are needed for callouts.