Thus, I believe that adding QA uploads to PAUSE would increase the average quality of modules. I haven't thought about implementation details, but I think the PAUSE indexer could simply index any upload of an orphaned module.

That is a nice thought and good idea

Nothing stops a PAUSE account holder right now from uploading a new version of an old module ... and naturally such UNAUTHORIZED releases won't make it to the index ...

but PAUSE indexer isn't the only game in town ... cpanm has http://cpanmetadb.plackperl.org/ ... you can create your own index of approved forks

So I agree with you an alternate index that includes latest UNAUTHORIZED release on CPAN culd be a good thing to have, a  cpan --latest-unauthorized or  cpanm --latest-unauthorized or  cpanp --latest-unauthorized option

So if you got internet website you can make an index today for use with cpanm --cpanmetadb ...

So if you got internet website you can make a mirror today for use with cpanm cpanm --mirror-only

proof of concept should be easy to write especially with api.metacpan.org ... example /search/reverse_dependencies/ExtUtils-MakeMaker / http://api.metacpan.org/v0/search/reverse_dependencies/ExtUtils::MakeMaker

similar thoughts and kinda related thoughts

Re: Howto abandon rights to a cpan module and stop getting bug reports for it? (The Lancaster Consensus: pause HANDOFF/ADOPTME /NEEDHELP ) / Howto abandon rights to a cpan module and stop getting bug reports for it?, PAUSE MINOS, Modules In Need Of Support), Fixing broken CPAN modules without author cooperation

There is always the nuclear option :) upload a fork (unauthorized version), Re: Fixing broken CPAN modules without author cooperation, Fixing broken CPAN modules without author cooperation, Why don't CPAN.pm and cpanminus update modules, even if they know about newer modules?