Unfortunately it looks as if the miiverse service will only allow you to talk to friends (with registered FC).

Not when the really-poorly-portrayed geeky guy says:
"Actually I think my
buddy OctoG123 has it figured out"
Then;
"Of course, what are
best friends for?"
Iwata confirmed personal accounts, plus the miiverse will have strangers, regional and worldwide.
FC's won't be used once Nintendo Network gets implemented for both the Wii U and 3DS, remember Iwata said that the 3DS will eventually get this kind of connectivity too.
He said that you would be able to see what other Miis are doing worldwide. I presume that is separate to your friends list that you can contact (which is obvious for video chat, the question is will it apply to comments too?)
Edit: Sorry, I wasn't really talking abut FCs. I more meant whether you'd be able to see updates from everyone playing that game rather than just your friends.
You will see your mii.
You will see all your friends miis (friends are people you add accounts, just like ps3/360. his friends account name is OctoG123, you can probably add friends in-game aswell like mariokart 7)
You will be able to see what everyone for instance in England (for me) is playing or has played. If a game is out it gets its own tile. peoples Miiverse app conversations are ALL presented in real time with fancy speach bubbles on your screen.
So if you publicly chat on Miiverse the public can see it.
but if you private chat with your friends, only your friends can see it
so to answer the question... BOTH. we will see real time talking from people we do not know. its like a facebook built into the WiiU. but we can hide conversations in a private chat.