I'm not quite sure what this field means , if it's supposed to be the first time the user actually uses moodle, or their account creation time.

At any rate, I have a bunch of mnet users (who have clicked around moodle) who have this set to 0.

I guess the solution is to make auth/mnet/auth.php set it when the user first lands, since if they're landing, they're accessing the site. However, a more robust fix might be to make something like user_accesstime_log check it and set it if it's not set.