I think what I'd like for that is a mono-repo with better support for subsetting instead of a multi-repo with better support for unioning.

I understand that Google (and may some of the other fangs) have tooling like that internally - but I haven't had the pleasure of using it.

That doesn't help if you actually need those repositories to exist separately.

For instance, consider the problem of having an external Open Source project your company maintains or heavily contributes to, and which also has common libraries shared with internal non-public work. Or, the same problem applies if you have a downstream Open Source project that needs to incorporate and vendor an upstream one but easily contribute changes upstream.

Some folks do this by generating the external repo as a filtered version of an internal monorepo, but that's awful in so many ways, and breaks the ability to properly treat the external repo as a first-class thing that can merge PRs normally. It leads to workflows where people submitting PRs feel like their work gets absorbed internally and maybe eventually spit back out the other end, rather than just being merged.