I've just had a giant Zara delivery (Mrs C-B is responsible for this new habit of mine, courtesy of a coat she once recommended) and the very lovely draped neck top here: www.zara.com/uk/en/woman/t-shirts/draped-top-c358008p1796531.html is actually in two parts - so there's a great big baby sized space between your covered tummy and the big drapey neck which would hide all action / activity.
NB it's a bit sheer, so you might not want to go for it immediately post partum unless you have a cardigan over the top!