Having someone else's Python installed, scares me (perhaps irrationally) and worries me that they're going to screw up the environment I rely upon for everything else I do, including my job. With that in mind, I use MacPorts Python (as a lot of folks do) and the three MacPorts py27-* libraries that are needed for Chirp.
My runtime is just a snapshot of a MacPorts install. You'd only use it for running CHIRP, nothing else.
So, your statement led me on an interesting night of converting GTK to run natively on quartz. I'm heading down that path now... thought it would be easier than it is seeming. But, sounds like that is probably the problem. :)
Yep, well, that's the price (pain) of wanting to do it yourself, but you should be able to make it work. It took me days to get it right, but I'm not a MacOS/MacPorts guy.
Note that if you use the GTK in your MacPorts for other things, there's a good chance converting it to Quartz will break those. You can either install a parallel version for Quartz support, or, you know, just install mine :)