Hehe, the amount of replies says a lot about the difficulty of seeing abs in k-dramas :D.
I'm no veteran, but from the ones I've seen, the only one that comes to mind is Oh My Venus, with a couple of abs scenes on the 16 episodes run, both by the main lead, and by his friend.
Also, in Her Private Life, there's a scene with the second main lead once by the end haha.
Both are rom-coms with some drama thrown in, though, so maybe the seconds of abs shown won't be worth it to you.