The developer definitely have ability and capability to implement this feature. But it's a HASSLE. Doing that (in my assumption) will involve extensive tweaking or even rewriting of the game engine if the engine didn't support it to begin with.
Your assumption is wrong - this isn't a feature of the engine. It's a matter of the animations and models used, not the engine used. Your entire post and argument is built around the assumption that it is something based on the engine used, so it's basically wrong from the beginning.