Not really thought about unity in particular, but we do have game ui as a potential use-case.
However, I imagine our performance at the moment exclude use in games. It's something that needs to be optimized.
If you want it to be used in games then on Windows, for example, Direct2D is the only reasonable option.
It will allow to render UI on layers inside DirectX 3D scenes: http://sciter.com/sciter-and-directx/