I was curious if you think the same method could be done with a GPS? You won't have to do anything except note the start time of each of your runs, then go home & download the data to your PC to see what the coast-down times are. Good (and free, which makes it even gooder) software that I found is SportTracks, which I talk about in my thread here:
Awesome free SW for tracking your efforts. You should be able to get fairly accurate coast-down data that way (certainly as good as the Instructables method).