I think this sounds like a fantastic and much needed idea! That being said, I'm not sure that it would sell well. My completely-not-backed-up-by-any-actual-data impression is that, in general, the crazy-making grammar abusers across the internet don't care that they're doing it badly, so they wouldn't be interested in improving. Although, if you found the right audience it could go really well (Students? Aspiring writers? Folks who are learning English?)
I think the potential is definitely there, you've just gotta find your niche.