I know that the GitHub web interface lets you search all repositories for files with a particular pathname (e.g. searching for path:/app/models/user.rb
yields >
GitHub introduced FileFinder in 2011.
Try it out: just hit t on any repo's file or directory view.[1]
So, You're still restricted to repository.
[1]https://github.com/blog/793-introducing-the-file-finder
Another approach to Your question:
Can I use Git to search for matching filenames in a repository?