Missing recommendations about how to increase participation in under-represented group.

I think you're really onto something. If we rated the experience level of a Wikipedian (1-5), and then rated the complexity of an article (1-5), we could figure out if they're in over their head. If experience < complexity, give them a nudge. We wouldn't need to necessarily restrict editing. We would just have to give them a reality check: it's harder for a newbie to add something to a featured article that sticks, compared to expanding a stub. We could even flag certain tasks, such as nominating an article for deletion or creating a new article, as being more "complex" tasks that might require a higher experience level to avoid getting "bitten".