When you create bleed for irregular shapes you will usually have a dieline or a similar object that defines the border of the shape that requires bleed.

However, if that is missing you may want to create such a dieline on the fly and you can use pdfToolbox' shapes technology for that. The attached Process Plan adds a dieline to the outer border of the shape generated from the combined objects on a page and adds bleed to it. The dieline is created as a spot color (Dieline) on a layer (Dieline) and the layer is associated with Processing Steps metadata (Structural:Cutting).

You can use the Process Plan with any PDF and any shape, but you may as well use this one.

Original

Result (Dieline has been colord pink)

In this article you can read what you can do when the shape has gaps.