First off, you should separate the patches for get_magic() and get_tag(). Second, why is there _get_pyc_magic_int() when it is never called? Third, all of this would be greatly simplified if you just had a _RAW_MAGIC_NUMBER of 3220, did the bytes object creation for _MAGIC_NUMBER in-place (i.e. no separate function), and then in the C code just got _RAW_MAGIC_NUMBER and did the MAGIC macro work there.
As for what is CPython-specific and what isn't, only the other VMs can state that officially, so I'm not going to worry about that yet (but I will ask before Python 3.3 goes out so as to minimize backporting patches in the future). But importlib needs to stabilize more before that can happen.