matching path rewrite without htaccess
If you have some control on the Aegir instance, and 100% trust your users that have sftp/ssh access to your site, you could use the provision_customhtaccess module:
https://github.com/mlutfy/provision_customhtaccess

This makes it possible to have a "htaccess-custom" file in your sites/example.org/ directory that will get included in your vhost configuration. However, it is a big security risk if you do not trust your users. They can close a vhost declaration, open a new one, do many bad configurations, etc.

Alternatively, if you're going to write a new module, it seems pretty easy to add in the match_redirect module. There is a feature request open, so I am sure the community would appreciate. Worst case, you can offer the maintainer to sponsor the development:
https://www.drupal.org/node/2118233

You can see how drupal_match_path() does a match and use that to rewrite the target redirect:
https://api.drupal.org/api/drupal/includes!path.inc/function/drupal_match_path/7