Writing a giant combo cPython interpreter doesn't sound like a winner to me.
The solution might be to convert cPython into IronPython everywhere and compile Python to .NET / Mono IL. Then Python3 could call Python2 code in the same way that C#, F#, C++ and Visual Basic can all call each other.
Or to be more acceptable to anti-Microsoft people, use the Parrot virtual machine, if that ever becomes usable. Another option might be LLVM. A problem with LLVM is that it doesn't specify a reflection, object, function and data sharing scheme in the same way that .NET does. Java might be another virtual machine that could be made to work.