And of course I figured it out 2 minutes after writing this message. For posterity, the syntax was close. It's actually like this:
elsif (NAS-IP-Address == "192.168.6.5" || NAS-IP-Address == "192.168.6.4") { reject = 1 } - List info/subscribe/unsubscribe? See http://www.freeradius.org/list/users.html