Z is well established as meaning "UTC time" <http://en.wikipedia.org/wiki/Coordinated_Universal_Time#Time_zones> so shouldn't be used for "zone not known."  rfc 3393 is clear that it's equivalent to +00:00.  

So the questions seem to be:
 * should there be an included battery to do this format at all?
 * should it represent utc as '+00:00' or as 'Z' by default - applications should have the choice.

It's probably reasonable to assume correct Python application code using datetime objects will know whether they have a local, utc, or unknown time.

The current patch does not seem to have any way to format an object with a declared UTC tzinfo as having a 'Z' prefix, which would be useful.