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The reason I marked this as a release blocker for 3.4 is because it's a key piece of functionality for writing command line apps which accept an encoding argument. I'll use "high" instead.
An interesting proposal was posted to the python-dev thread [1]: using self.detach() and self.__init__() to reinitialise the wrapper *in-place*.
With that approach, the pure Python version of set_encoding() would look something like this:
 _sentinel = object()
 def set_encoding(self, encoding=_sentinel, errors=_sentinel):
 if encoding is _sentinel:
 encoding = self.encoding
 if errors is _sentinel:
 errors = self.errors
 self.__init__(self.detach(),
 encoding, errors,
 self._line_buffering,
 self._readnl,
 self._write_through)
(The pure Python version currently has no self._write_through attribute - see #15571)
Note that this approach addresses my main concern with the use of detach() for this: because the wrapper is reinitialised in place, old references (such as the sys.__std*__ attributes) will also see the change.
Yes, such a function would need a nice clear warning to say "Calling this may cause data loss or corruption if used without due care and attention", but it should *work*. (Without automatic garbage collection, the C implementation would need an explicit internal "reinitialise" function rather than being able to just use the existing init function directly, but that shouldn't be a major problem).
[1] http://mail.python.org/pipermail/python-ideas/2012-August/015898.html