Since you mention being cut off at an angle I'm going to assume that you don't actually have a half circle there, but rather a quarter-circle or something similar, because using a 180 degree fill on a half circle would cut it at exactly the right angle. Unfortunately there is nothing in NGUI that would let you have some arbitrary cutting out of the box. You can write a custom fill option by deriving either from UISprite, or -- more easier -- from UITexture and then writing your own OnFill function.
In Windward I had curved health bars on ships (about a 30 degree bend), but I wasn't doing anything fancy with them. I was simply using a vertical fill and yet many assumed I had some curved fill when I didn't.