We actually did this. When I started at Uber ATG one of our devs made a submodule called `uber_monorepo` that was linked from the root of our git repo. In our repo's `.buckconfig` file we had access to everything that the mobile developers at Uber had access to by prefixing our targets with `//uber_monorepo/`

We did however run into the standard dependency resolution issue when you have any loosely coupled dependency. Updating our submodule usually required a 1-2 day effort because we were out of sync for a month or two.