Sorry, you’re right, I missed that this was for install. This will no longer be possible in future (at least, not using pip). All installs will be done by building a wheel (or using a pre-built wheel) and installing that. There’s no option in pip to customise the install process (how to install a wheel is defined by the wheel specification, and we follow that spec).
If you want to do custom actions as part of installing a package, you’ll need a different installer, I’m afraid. (Which probably means writing your own, although that could wrap pip if you wanted, to handle the actual wheel install).
Edit: Note that setuptools have deprecated the setup.py install command, so pip really has no choice other than to drop our usage of it, even if we wanted to do otherwise (we don’t, though - we’re removing all code that special-cases setuptools in favour of using the standard build backend interface).