AFAIK, the converter attribute is only to "be set" with a function of such signature. It is not documented that it can be used and it is only used on the format time function.

The only situation where this might break someone is if they inherit from formatter, substitute the formatTime function by their own implementation and rely on the function returning a time tuple. Doing that is relying on the implementation of the formatter though, as the docs explain is just something to be set.

This could be released as part of a new Python version, I'd be surprised if this is an issue and it will improve quite nicely the way users can configure the formatting of timestamps.

Even if adding it to the cookbook would "work" I think it would be much better to have this change and provide a nicer experience with the default class.

This is your call though! But I'd say it would be quite a nice improvement, was speaking with some other devs at PyCon and they seemed quite excited.