Is it possible to use an instance of StageWebView (H.264 video) as a texture on an object in Away3D 4.0?

Do you really mean StageWebView (which is a WebKit implementation unless I’m mistaken) or do you mean StageVideo (GPU-accelerated H.264 decoding)? Anyway, there is no way of retrieving the output generated by any of those APIs and using it as a texture, I’m afraid. At some point we expect Adobe to enable at least access to the StageVideo output as textures, but it’s not in any of the released players.