In the Perl world (and possibly others), the CPAN maintainers will transfer ownership/maintainership privileges to you if you can prove you haven't been able to contact the original author (typically by sending an email and CC'ing the perl5-modules mailing list). This works well when there is one or two distributions that are the canonical way to solve a task.

Obviously not possible for GitHub as a whole, but perhaps we could reimagine how GitHub interacts with specific language/tool/whatever communities.