Is it possible to apply a mask render.RenderView displayed on a players HUD?

Basically I want a view rendered to the players HUD, but with a mask applied to it so that it’s circular rather than rectangular.

Is this even possible? or would I have to use a custom RT texture?

You may want to look at the radar made by Hurricaaane or Ha3. I know he renders his radar using a render-target to a rounded shape.