Does Unity include its own Python interpreter and does it use it for post process builds on Mac?

It depends on just how much Unity have modified MonoDevelop to make their editor(s). If they haven't done much to Python then there should be an ironpython/ directory in /Library/Frameworks/Mono.framework/Versions/Current/lib/ which it will use by default (via the /Library/Frameworks/Mono.framework/Versions/Current/bin/ipy* scripts). This can easily be overridden with an alternative installation of IronPython or even the Python for .NET code to integrate Mono with the native Python installation. Both IronPython and Python for .NET can play nicely with Python 2.7 (current releases of IronPython are re-implementations of Python 2.7).

Admittedly I've only been looking at this project for a day or two, but I've got Mono to play with IronPython 2.7.4 and 2.7.5-beta2. I've also got Python for .Net to see my install of CPython 2.7.8 (just the standard installer from python.org). Unfortunately neither Python for .Net nor IronPython have completed useful ports for any of the Python 3 releases. Also, PyPy have not yet completed their own .NET/Mono porting, but when that completes or reaches a usable state it's likely to make some things a little more interesting.

As for the specifics of what Unity have done, they appear to have the framework data stored inside the app instead of in the OS X /Library/Frameworks/ directory. So you'll need to either use the show contents option in Finder or just cd through the the app subdirectories in Terminal. If, like me, you just copied the whole Unity folder into /Applications then you're looking for something like this (note: version numbers are likely to change depending on the Unity release used):

/Applications/Unity/MonoDevelop.app/Contents/Frameworks/Mono.framework/Versions/Current/lib/

The directory structure is identical to a standard Mono installation, just kept inside the app directories. So the methods of installing, adding or updating Python to Mono will work here too. A quick update for what I've got in Mono to Unity results in this:

 bash$ pwd
 /Applications/Unity/MonoDevelop.app/Contents/Frameworks/Mono.framework/Versions/Current
 bash$ ./bin/mono lib/IronPython-2.7.4/ipy.exe
 IronPython 2.7.4 (2.7.0.40) on Mono 4.0.30319.1 (32-bit)
 Type "help", "copyright", "credits" or "license" for more information.
 >>> quit()
 bash$ ./bin/mono lib/IronPython-2.7.5b2/ipy.exe
 IronPython 2.7.5b2 (2.7.5.0) on Mono 4.0.30319.1 (32-bit)
 Type "help", "copyright", "credits" or "license" for more information.
 >>> quit()
 bash$ ./bin/mono lib/pythonnet/python.exe 
 Python 2.7.8 (v2.7.8:ee879c0ffa11, Jun 29 2014, 21:07:35) 
 [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
 Type "help", "copyright", "credits" or "license" for more information.
 >>> import sys
 >>> print sys.platform
 darwin
 >>> sys.exit()
 bash$ 

Looks fine. User friendly plug-in/add-on/extension support within the Mono/Unity GUI is up to the individual. Note that the Mono developers disabled the Python add-on for OS X so they may have broken something within their GUI in relation to Python. I'm not sure, I haven't tried getting around that yet.