As well as the main libraries in /usr/share/gir-1.0, a number of packages install gir files in other directories, for example GEdit. They all seem to use the pattern /usr/share/*/gir-1.0, where * is the package name, so it should be quite easy to scan for them, although I don't know what extra steps might be needed in case fakegir needs to use the corresponding typelib files in /usr/lib/x86_64-linux-gnu/*/girepository-1.0/.
Should I try to make a simple implementation and raise a PR?