I have no thoughts about giving transparecies to a National Park. There was an article sveral years ago in Photo Techniques magazine about donating work to charitable orginations as being good self promotion. I have not had any sucess with it myself since no one I have ever met would even take one of my photos for free because when I made such an offer they have always told me my prints are too expensive.

I think, if nothing else, you will find satisfaction in having your work put to a good use and which will inspire park visitors.