I would be really nice if this simple text field were added if someone includes the 'Guide to Pathfinder Society Organized Play' book. This could sit on the summary tab or description tab.
You want an ID under an ability? Otherwise you're looking at a cross request. One Data to create an ID, the other to place the ID somewhere other than just under the Abilities. Doable, but time consuming.
I guess I was thinking it would go on the Summary tab beneath the 'Player' field in the 'Character Basics' section.
Then that is a Code Feature Request involving the UI. Along with an Output Token to export that value.
I just type in my id number in the player name field.