I would like to be able to use a virtualenv with --no-system-site-packages as the parent for another virtualenv with --system-site-packages. The child virtualenv would find all the packages from the parent.
You're certainly welcome to work on this, but I think it will be non-trivial. I recommend hitting your use case by compiling a clean Python and making virtualenvs from that.
I will probably wind up compiling an actualenv, but setting PYTHONUSERBASE to the "middle" Python does almost what I want (but it includes the real system Python, too).