Don't want to pour cold water on the idea, just maybe tweak it a little. 1st. nothing brings out idiots like competition, and I absolutely hate fishing when there are derby's or tournaments going on. The competitors think they own the water. 2nd. If you don't have an official measuring station, honesty will become an issue. 3rd. if you're trying to catch and release, taking measurements only increases the handling and prolongs the release.
Kind of sounds lame, but it's the only alternative I could come up with. How about a best photo competition. Doesn't have to be with a fish, can be anything related to the event.