Requiring GPL on derivative works is the one-way boundary I mentioned; the simulated "copyright-free zone" in question is comprised of all GPL-licensed software

Requiring GPL on derivative works is the one-way boundary I mentioned; the simulated "copyright-free zone" in question is comprised of all GPL-licensed software, so anything existing wholly within that realm isn't impacted. The purpose of the derivative license requirement is to discourage external dependencies creating copyright-by-proxy inside the GPL zone. I don't actually think the source code publishing requirement is necessary; something like CC BY-SA would probably suffice.