The article says the new style says that you can use either a macron or a doubled letter, but it's not clear if that's supported for keyboard input on various platforms.

But in the case of ō, you can only use a doubled letter if the underlying word is おお. If it is おう then you don't have a doubled letter you can use; you need "ou" and that's not Hepburn any more. It is "wāpuro rōmaji" (word processor romaji).