> sbt, the bug is not that the encoding is inefficient. The problem is we 
> cannot unpickle bytes streams from Python 3 using Python 2.

Ah.  Well you can do it using codecs.encode.

Python 3.3.0a0 (default, Dec  8 2011, 17:56:13) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle, codecs
>>>
>>> class MyBytes(bytes):
...     def __reduce__(self):
...         return codecs.encode, (self.decode('latin1'), 'latin1')
...
>>> pickle.dumps(MyBytes(b"hello"), 2)
b'\x80\x02c_codecs\nencode\nq\x00X\x05\x00\x00\x00helloq\x01X\x06\x00\x00\x00latin1q\x02\x86q\x03Rq\x04.'

Actually, I notice that array objects created by Python 3 are not decodable on Python 2.  See Issue 13566.