Does the Captive Portal work without an Internet connection, that is, without real external DNS?

It looks like the way the CP works now, a URL has to resolve to a real IP before it redirects to the
splash screen.  I'm trying to use the Captive Portal to forward all users to a local web server in a
setup without Internet access.  But in this particular configuration, with no real external DNS,
clients get a page not found error instead of the captive portal screen.  Local sites that are
explicitly listed in the DNS forwarder work, but not general public sites, like google.com. 

If this is the way the captive portal is supposed to work, is there another way to redirect all
local traffic to a local web server?  The clients won't ever actually go to the URL they put in, but
I'd like it so that if they type in, say, google.com, that they are forwarded even though that site
is currently unavailable.

Thanks for a great product,
Nate