A tartan for National Park Service employees, their children, and park volunteers and enthusiasts is a nice idea. It would be terrific if the tartan was official but it doesn't have to be. The US Air Force tartans are unofficial although they either were or are currently worn by US Air Force pipe bands.

As a US Forest Service brat I can say that the USFS had a big impact on my upbringing as we moved from forest to forest. While my father's training was in recreation he spent a lot of time on forest fires and his last post was at the Boise Interagency Fire Center. I have a firefighter memorial tartan kilt on order.

It might be easier to go through a park volunteer organization. In some instances a single branch of an organization has registered a tartan and then suggested it is open to all to wear. Good luck with your project.