Instead, I'm starting to think that with the right error message,
"Explicit is better than implicit" and "In the face of ambiguity,
refuse the temptation to guess" should rule the day here.

For example:

$pip3 install Django
pip is not currently installed. Run 'pip3 bootstrap' or 'pip3
bootstrap --system' to install it.
$pip3 bootstrap --help
Usage:
pip3 bootstrap
pip3 bootstrap --system

sounds reasonable to me.

Linux distros could then patch the 'pip3 bootstrap --system' variant
to invoke the appropriate platform specific installation command.

well, e.g. debian patches distutils.command.install with it's /usr/local layer and dist-packages patches.

I imagine, those kind of patches would also occur in distlib as well now? (which will be handling the bootstrap installs)

Marcus