Are there instances in which an entire sentence constitutes a focus--new information?

Yes and no. Here's an utterance that is all new. But is it a sentence? It all depends on your perspective. I'd argue that as soon as you have a complete clause with an analytically articulated and semantically relevant subject and predicate, you're doing something to mark the topic or focus (theme/rheme). Because you're always doing something to place it into the discourse structure.

The known/new information approach is only an analogy to the whole point of topic/focus. So while thetic sentences like 'My girlfriend left me' may seem like they're providing all new information in the discourse context of 'Why are you crying?', they're already involved in marking the known aspect of 'X is known to have a girlfriend' and the new aspect 'girlfriend left X'. In Czech, this may be done through word order and/or prosody and in English through definiteness and/or prosody (plus a bunch of other discursive ways).

So you're left with sentences like 'It's raining.' or 'It's cold.'