This is simply a personal exercise/project, I don't intend to inflict this on the world. My goal is to further my understanding of both languages, and it would be a nice having a nice Python code base in the end. 
But basically there is a program I like with a large (about 100ksloc) C++ code base, what I'd like to do is gradually rewrite modules in Python while having the existing C++ code use the new Python. The reason I'd like to do it this way instead of starting from scratch is because the application would/should be fully functional from the beginning. 
Prime question is: Is this realistic? I'm pretty sure it'd work in theory, but in practice is it just not worth the effort?
Has anyone done it before?
having the existing C++ code use the new Python

I think you'll find that python makes a better glue language for C++ than C++ does for python.  So you probably will have an easier time converting the main function to python first (in fact, the first step could be just renaming the main function, compiling the C++ app as a python library, and having one line of python which calls the renamed main).  Then start moving functionality from C++ to python one line at a time.
As @freedompeace suggested, you may want to leave significant chunks in C++, with python providing the glue.  Depends on whether portability or speed/memory efficiency is more important to you.
Is gradually translating a program written in C++ to Python feasible?