This already happens now. Go look through a few of the "Show HN" authors - you'll inevitably see around several accounts that are 50-100 days old with a karma of 1 to avoid a green label.

The OP is talking about posts, not comments. The simplest solution might be to prevent someone from posting a "Show HN" until they’ve earned twenty-five or fifty karma, to demonstrate that they’ve been actively participating on Hacker News rather than using it solely to promote themselves.