I like "as", reads just right. foo.IWrite::write(v) seems all wrong -- foo::IWrite.write(v) seems much better. But, wait, that matches my dislike of how you wanted to use : in path expressions too -- I guess I think of :: as a harder separator than you.
But it doesn't really matter, "as" is perfect. Of course, introducing syntax for a feature that doesn't have a canonical or standard implementation might be premature ;)