Given the vast breadth of the code under //src/lib/ and the project's desire to encourage more local ownership, there is intentionally no //src/lib/OWNERS. Rather, the addition of new subdirectories is expected to fall back toward requiring an OWNERS override in the following way.
The initial commit of //src/lib/foo should be limited to an OWNERS file, as well as possibly a README.md. The override should be requested on this change, in turn limiting the sign-off to the high-level intent of the new codebase and its desired set of maintainers. Once those maintainers are in place, contributions of actual code can proceed as usual under their direction alone.