It could probably be changed to copy libs/$LIB/config as a special case.

Actually, bcp already has a list of things it can't figure out automatically: https://github.com/boostorg/bcp/blob/develop/add_path.cpp#L195 which would need to be updated with, I assume, std::pair<fs::path, fs::path>("libs/log/build", "libs/log/config"), and std::pair<fs::path, fs::path>("libs/test/build", "libs/predef/check"), or something like that.