Set program zero using XY positioning laser

A question associated with this project to integrate a crosshair laser into my machine.

At present the crosshair laser has been successfully integrated into the gvcp panel. My next effort is to allow the user to select the coordinate system to set to zero given the position marked by the crosshairs on the stock. I've added a HAL_Combo box which I can select the coordinate system, however, while monitoring the s32 pin with show_configuration I do not see any change in the value of the pin when selecting a different value. I would like to reference both the label and value columns of the liststore associate with the selection. Is that possible without resorting to a handler?