It appears that the 'parent' => '@aliasname' option does not work when @aliasname is defined in a separate alias file. Aliases from this file do work when called directly.
Is this the case for others, and if so is it intentional/necessary or a bug?
(Background: My goal is similar to what i was after in main Drush settings centrally handled with the project, while what is person-specific - for instance, remote-user - can be overridden locally.), with a different approach: having a project's