It seems like we're all stuck on that missing CG because no one seems to have an answer no matter how much we ask :/ I think it involves Haato in a maid costume because I saw it in one of the Twitter CG sneak-peeks months ago, but I've yet to find a way to trigger it. I tried replaying with Toru as a dominant top and then spamming the maid costume but it didn't seem to work.
It is kinda hidden, but basically, you need to take the cross-dressing job when Toru is at the agency. You then need to win the job. Then, after you become lovers, if you have maid costume with M size (Haato's size) then you can get the scene... if I remember right. I'm not in front of my work laptop right now.