Can you wear a pocket square in the breast pocket of your overcoat? I suppose you can, but I don't recommend it. However, this doesn't mean you have to keep your favorite coat's breast pocket empty.
Instead, throw your gloves in there when you are not wearing them, i.e.:
photos by Scott Schuman (pictured directly above)
I think this is such a chic look, and one that I can shamefully admit I have copied, i.e.:
Thanks for reading.
Yours in style,