Reading the original issue that added itms-services support to url splitting and joining, it does make me wonder if it would be worth changing urllib to read the initial setup of its module level attributes from a JSON config file bundled with the standard library rather than hardcoding its knowledge of all the relevant schemes.
Then the bundled app generators could just drop itms-services from that config file rather than having to patch urllib.py directly.
As a more general case though, I think it does make sense to view this as equivalent to Linux distro patching (even with the URL schema config file idea, the bundled app generators would still need to modify the config file).