No, the Mac does not have a Windows-style Registry database. But, your registry access will work on the Mac. But these are only useful as long as they reference configuration options that your application uses. The registry classes will not provide any useful information about any operating system configuration or settings.
/Library/Application Support
I’m sure there is. But this isn’t anything I can help you with.
I don’t know enough about what kind of plug-in you have and how it was developed to comment on this. More information would be helpful. But in general, scripts written in Python should be very portable. You may run into platform-specific issues, though.
I would recommend against using registry type functions on the Mac if at all possible. You can typically get away with the same same functionality by reading/writing a file that you would save somewhere like the /Library/Application Support/McNeel directory that Dale mentioned.
Could you explain a little bit more about what you need registry access for? We may be able to write some convenience functions for you to make things easier.