Is there a stand available so you could set the 520/521 down on a desk or counter in front of a mirror and have the camera angled correctly so your arm doesn't get tired from holding up the phone? I could imagine either using a bathroom mirror or having a small mirror sitting on a counter with the phone camera pointed towards it. You could watch the other person on the screen by looking at the reflection of the phone screen in the mirror.
I know Vainworks makes a perfect stand for video chats on phones without front facing cameras with a mirror built in, but it won't work for the Lumia 520 or 521 because it only works for phones with the rear camera near the side of the phone and the Lumias have the camera in the middle.