How can I make my Apple TV 1st gen join a WiFi network without having a TV connected? Is there an OSX utility for set up in the same way there is with Airport Express?
No. You need to connect the AppleTV to a video output of some sort, otherwise there's no way to specify the SSID (the WiFi network identifier) and the password (if any). After you've done that once, you can unplug it from the video output.