It's my impression that hardening off involves only a few hours of outdoor exposure at a time, which you can achieve by raising the lid to your cold frame and closing it back up again. I feel that a direct transplant to a covered row might not qualify as hardening off and you might experience some stunt plants... If you have your coldframe already set up, why not try 50% the old way and 50% the other. If your climate is right, you might be able to get away with it with little hindrance. Otherwise, I would advocate for starting seeds directly into the soil and skip the indoor starts if you want to skip cold framing.