Substitute or Add a Timeline Milestone Symbol

[RSWallace]

I am automating generation of a swimlane/timeline chart in Visio 2007 and nothing will do but a faithful reproduction of the original (OMG hand-drawn in PowerPoint) symbols, one of which is a milestone represented by a 5-pointed star symbol. Visio offers 9 symbols ("Circle;Cylindrical Line;Square;Diamond;Line;2 Triangle;Triangle;X;Pin"), but no star.

I am pretty comfortable using "Edit Master" and wandering around in its ShapeSheet, but I don't see how to change symbols. Inferring from the few differences between the ShapeSheets for "Diamond" and "Triangle" suggest that the symbol is specified by selecting an element form the above list, but I cannot determine where those elements are defined. Also, no obvious "symbol number" or similar references.

The obvious approach of ungrouping the elements of the Master and substituting another symbol mysteriously destroys the desired milestone behavior. Any help or insight would be appreciated.