FYI It's not based on Qt but pure Symbian C++ code.
Build they released is ancient. Somewhere before Symbian Anna code and definitely not optimized for Symbian Belle.
Some of us discussed it with Viber and they aren't interested at all. No improvements for Symbian yet alone a build for Harmattan.
If you want it so badly, you'll have to reverse engineer it and build it on your own from scratch.