Multi-sites local dev alias question for Drupal 6 / OS X
Seems like what you want could be accomplished by creating a symbolic link in the sites directory. Linking to subsite.com by the name subsite.dev would do it:

$ cd [sites-directory]
$ ln -s subsite.com subsite.dev


Mind you, you might need to change the $conf['cookie_domain'] and $conf['base_url'] values in the settings.php file if you want to be able to log in to your local dev site.