This may not be the best answer, but I would do this by making a document property and then referencing the property in the footer using the Fields menu.
- Make the Property by going to the File->Properties menu and selecting the Custom Properties tab. Add the title of your custom property, ID in your case.
- Add the Field to the document footer by placing the cursor in the footer area. Go to the Insert->Field->More Fields menu (or use the keyboard shortcut Ctrl-F2). In the Fields panel select the DocInformation tab. There should be a drop-down indicator next to Custom in the Type column.
This will insert the ID on every footer, but will require the user of the document to go to the document properties to indicate their ID.
Hope this helps,
hill_hobbit