The AOPA chit route could work. Consider the AOPA Aerobatics Certificate: this is unofficial, as no special licence or rating is required to fly aeros, but over the years the AOPA course has gained customer recognition as the standard intro to the discipline. A GPS course, sponsored by AOPA or someone else, could acquire similar kudos over time. I would expect to see GPS-nav being incorporated into the PPL syllabus within a few years in any event.