Too bad. Honestly the questions annoys me. It comes from experience training hundreds of clients, not everything can be supported by 20 peer-reviewed studies. Why not try it and see for yourself if and how it works?
Abs have a much lower strength potential and other muscles, you will not create a lot of muscle damage and as such they can be trained more frequently than other muscles.
And most people have really bad mind muscle connection with the abs (especially those with a desk job who spend more of their day seated). As such not only CAN you train the abs more often, you SHOULD train them more often to improve the mind muscle connection and become better at recruiting them.
But it has been my experience that when you train a muscle with the same isolation exercises (training the "6 pack" is pretty much always doing the same movement pattern) VERY frequently you lose the quality of contraction after 2-3 weeks. Why? I don't know exactly, but it has just been my experience. When training abs daily after 2-3 weeks you lose the quality of contraction, you feel your abs less. But for the first 2-3 weeks it improves. After a few weeks with no direct abs work they become responsive again.. i cannot explain why but it has been my experience that it does.
I this had my best results (and with clients) doing abs every day for 2-3 weeks then 2-3 weeks without direct abs work (my abs still get some work done because I do loaded carries, squats and deads).