MAL, I think an idea like this is good, but I have one concern. Let's say Python 2.3 comes out with email pkg 2.5. Later I find some bugs and release email 2.5.1 and also fix the Python 2.3 maint branch. Now Python 2.3.1 comes out.
At best the email package in system-packages should be ignored. At worse, it /has/ to be ignored. E.g. Python 2.3.2 comes out with email 2.5.2.
My suggestion is that the Python micro-release number be included in the path to system-packages. IOW, system-packages must exactly match the Python version number, not just the maj.min number.