It appears you are right clicking view source in the custom property in the UI.  Copy data_path will give you the path,  eg. `["prop"]` where the property name is prop.  In the material properties panel the rna_item will be the active material, in the object, the context.object, in the data `context.object.data`, in the `scene context.scene` etc.

The code you are seeing is setup to handle the different object types that can have a custom property.  `escape_identifier` most likely handles garbage property names.

The pie menu template.  A custom property "prop" has been added to the scene.  The 'rna_item' equates to `context.scene`, the key to "prop":

    class VIEW3D_PIE_template(Menu):
        # label is displayed at the center of the pie menu.
        bl_label = "Select Mode"

        def draw(self, context):
            layout = self.layout

            pie = layout.menu_pie()
            pie.prop(context.scene, '["prop"]')
            # operator_enum will just spread all available options
            # for the type enum of the operator on the pie
            pie.operator_enum("mesh.select_mode", "type")