“Complete The Map Project” (with a specific reasonably limited goal)?
Because the images will end up on OpenStreetMap, why not create/add an OSM email list for the contest? Or, the contest could become a Maproulette challenge. For those who don’t know, Maproulette is a way to focus on a specific feature of the OSM map that needs to be fixed, such as unmarked level crossings for railroad tracks. Maproulette presents the items that need to be fixed, one by one. The person working on it can fix it or go on to the next one, whatever they choose. Blank spots on Mapillary for Route 66 could be the challenge. While, unlike most Maproulette challenges, you couldn’t fix it right away–you have to go out and get the images–but it could be set up so you could come back later and mark it “Fixed” when you upload the images. Just a thought.
One point, I do like your way of thinking, only change I’d make is not to just fill in the gaps, I think better would be to cover the entire route anew.
Just my thought.