If we go with separate repos, then we'll need Google's repo tool, use git-submodules, or some other solution. The binaries have been created by a third party,

If we go with separate repos, then we'll need Google's repo tool, use git-submodules, or some other solution. The binaries have been created by a third party, and can't necessarily be built by us. I haven't checked in all cases, the codebase is a twisty maze of different build systems, not all integrated.