Pfft.. too little, too late. JPEG is "good enough" and I don't want a huge clusterfuck of incompatibility problems with my libraries.
In terms of widespread adoption, I think you're right, Joe's Image Viewer is unlikely to ever come with Lepton support. But I wouldn't dismiss this so quickly, as large sites might force the issue into the browser space.
Take Facebook as an example, think of the trillions of photos they store (they claim 2 billion are uploaded each day). Facebook archives older, infrequently-accessed photos to Blu-Ray and has an army of jukeboxes ready to swap in discs when someone actually tries to load that family reunion pic from 8 years ago. Gaining another 20% on compression means not just 20% less live storage, but also 20% fewer optical discs, 20% smaller backups, 20% fewer disc-swapping robots, 20% less square footage to lease and cool... We're talking millions and millions of dollars in savings. Facebook would be stupid not to hand Mozilla a chunk of that money and say "Lepton, implement it." Google and Microsoft would realize their own enormous cost savings by putting Lepton capability into their respective browsers.