Code for create the Ground Guard around the board outline

There is already a standard function in Allegro which can add vias around the board outline. It is in the Place->via array menu.

If you want to make such function then I would start with something like this:
get with axlDBGetShapes the outline shape (If using 16.x then I hope you have a single shape as an outline... in 17.2 this is the only way to draw a outline. )
Then convert the shape to a polygon with axlPolyFromDB and use axlPolyExpand to shrink the outline.
To create a Cline on the via's you will need a rpath. However there is no standart skill function to do this... The axlDB2Path() only takes a shape as input, not a polygon.
You can do two things;
- create a temporary shape from the contracted polygon and use the axlDB2Path() to get it's path. Once you have the path then use axlDeleteObject() to remove the shape.
- make a function which converts each poly vertices to a rpath.

Once you have the rpath of the contracted outline you can use this to create a cline on an etch layer with axlDBCreateLine()
For placing the via's you need to determine points along each segment of the rpath. For straight lines you can use axlMUniVector() to calculate a point on the line.
For arcs this a bit more complicated, but with some math you can solve this.

Bram