I don't really know that much about pickle, but Antoine mentioned that 'bytearray'
works fine going from 3.2 to 2.7.  Given that, can't we just compose 'bytes' with
'bytearray'?  Something like:

Python 3.3.0a0 (default:aab45b904141+, Dec 10 2011, 13:34:41)
[GCC 4.6.2 20111027 (Red Hat 4.6.2-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
...
>>> class Bytes(bytes):
...    def __reduce__(self):
...       return bytes, (bytearray(self),)
... 
>>> pickletools.dis(pickle.dumps(Bytes(b'abc'), protocol=2))
    0: \x80 PROTO      2
    2: c    GLOBAL     '__builtin__ bytes'
   21: q    BINPUT     0
   23: c    GLOBAL     '__builtin__ bytearray'
   46: q    BINPUT     1
   48: X    BINUNICODE 'abc'
   56: q    BINPUT     2
   58: X    BINUNICODE 'latin-1'
   70: q    BINPUT     3
   72: \x86 TUPLE2
   73: q    BINPUT     4
   75: R    REDUCE
   76: q    BINPUT     5
   78: \x85 TUPLE1
   79: q    BINPUT     6
   81: R    REDUCE
   82: q    BINPUT     7
   84: .    STOP
highest protocol among opcodes = 2
>>> pickle.dumps(Bytes(b'abc'), protocol=2)
b'\x80\x02c__builtin__\nbytes\nq\x00c__builtin__\nbytearray\nq\x01X\x03\x00\x00\x00abcq\x02X\x07\x00\x00\x00latin-1q\x03\x86q\x04Rq\x05\x85q\x06Rq\x07.'

[meadori@motherbrain cpython]$ python
Python 2.7.2 (default, Oct 27 2011, 01:40:22) 
[GCC 4.6.1 20111003 (Red Hat 4.6.1-10)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
...
>>> pickle.loads(b'\x80\x02c__builtin__\nbytes\nq\x00c__builtin__\nbytearray\nq\x01X\x03\x00\x00\x00abcq\x02X\x07\x00\x00\x00latin-1q\x03\x86q\x04Rq\x05\x85q\x06Rq\x07.')
'abc'

If this method is OK, then the patch is pretty simple.  See attached.