What is not clear to me after looking around, is how I upload the new tarball so it will show up within the original (awol) author's directory.

That won't happen, and it doesn't need to happen, you have taken over maintenance ( takeover, takeover ), the hard part is over :)

Because of co-maintenance he who uploads last/highest version number, wins :)

https://metacpan.org/release/Wx always takes you to latest version

cpanp -i Wx always installs the latest version

http://search.cpan.org/~mbarbon/Wx makes it clear its not the latest version

https://metacpan.org/release/MBARBON/Wx-0.98/ and http://search.cpan.org/dist/Wx both show a link to the latest