Does drupal_get_path_alias only work if you are requesting a page, or can it also work if you are requesting a form?
For Drupal 6 there was a Sub-path URL Alias module.


  The Drupal Path module matches only full URLs when creating SEO-friendly aliases. This module extends that behavior by also matching known sub-paths and replacing them with their respective alias. For example, if node/1 is aliased to content/alias, this module rewrites the link to the node edit page node/1/edit to use the aliased URL content/alias/edit instead. This also includes Views URLs taking a node as argument (node/%/yourview), in short, every URL that is based on, or extends, an existing alias. In combination with the Pathauto module it is possible to get rid of all remaining exposed internal URLs.


I'm not sure if there is any for D7. If not porting it would be the option to go for.