Thank you for the advice. I'll look at hardwiring something in /etc/inc/.
I realize that mixing subnets is not the ideal situation, but I hesitate to buy extra hardware to accommodate the 10 or so visitors per year who come to the office and ask if they can hook up their laptop.
I also think it's rude to say "NO you can't hook up your laptop," since they let me hook up my laptop when I visit their office.
Assigning visitors an IP address on a different subnet with stricter rules is a cheap and easy way to avoid problems like visitors being able to browse our samba shares. It also would prevent problems like we had when a temp that we hired to help us with filing happened to have a torrent client running in the background on his laptop, and we got a DMCA notice because of an illegal download. Since only 5 or 6 ports are open from that subnet to WAN, the torrent client wouldn't have worked.
I know it won't stop the NSA or a malicious hacker, but that's not the kind of people we invite into our office anyway.