I think the cleanest fix is:

• in the Engine, call the parameter authors instead of commitAuthorPHIDs;
• pass the readable form here: '/audit/?authors='.$user->getUsername()

This is clean and hints to users that they can construct their own similar query in a sensible way (authors=username).

That is, call the HTTP parameter authors. The storage parameter can/should remain commitAuthorPHIDs. For example:

$saved->setParameter(
  'commitAuthorPHIDs',
  $this->readUsersFromRequest($request, 'authors'));

The narrative here is something like "authors is in a human-preferred format, commitAuthorPHIDs is in a normalized computer-preferred format".

WMF also has some issue with their markup where [] is unreasonably difficult to type in URIs:

http://fab.wmflabs.org/T88

This isn't really a problem on our side, but since the authors=epriestley form is better for humans in general we avoid that issue for free.

Cool, looks great.