This is a great idea. The suggestions in your first draft would help clean up some of the uglier corners of the PyCXX code.
I'd suggest that you might want to add at least 1 PyCXX based extension to your testing. PyCXX aims to expose all the C API as C++ classes. (I'm missing the class variable support as you discussed on this list a while ago).
I'd be happy to support your API in PyCXX.