A little googling reveals a few FreeBSD captive portals:
      http://www.cc.saga-u.ac.jp/opengate/index-e.html
      http://opensplash.qalab.com/
      http://software.stockholmopen.net/index.shtml
and a bunch of others (not necessarily FreeBSD based) at:
     http://www.personaltelco.net/index.cgi/PortalSoftware

Can someone identify a good starting candidate? (Yes, at least one of these
relies on Perl and another is C based.  And they mostly include
authentication
which we don't need.)

I found a couple of FreeBSD folks in our local Wireless User Group
(www.sdwug.org) who have said they can help out, but I think if someone with
some m0n0wall knowledge can offer a starting direction it will help to get
them going.

cheers, michael