Anyone Make an Off the Shelf DHCP Proxy NAT / Firewall?

I'm looking for a commodity small LAN firewall / NAT box that would support
a DHCP proxy feature on its *external* interface, and then send those DHCP
requests to a specific host behind the arp.

My application is that I have a Windows 2003 domain controller that I would
like to have act as a DHCP server for the computers in its domain. The
clients would be in front of a firewall / NAT and the domain controller
would be behind the firwall / NAT. I want to find an NAT that would
present a DHCP proxy on its external interface and send those requests to
the domain controller.

The existing firewall we have in place supports DHCP proxy only in a weak
way. It also requires adding some additional software to the firewall that
I don't particularly want running on the firewall. So I would run the DHCP
proxy as a bypass to the existing firewall, just for DHCP only. No other
traffic besides DHCP would go in or out of the new firewall.

I see that Netopia's Cayman 3500 series supports a DHCP relay from
*internal* machines to an outside DHCP server. Unfortunately, that's the
opposite of what I need, unless I am ready to turn the firewall 180 degrees
around and make the clients the internal network and the domain controller
the external. That would require some very iffy configuration to make
secure, if it could be done. My preference is to find a similar
off-the-shelf product that already incorporate DHCP proxy as I describe it.
I would prefer to not have to install UNIX and do anything custom. Due to
time limitations, off the shelf is what I need.

Does this product exist?

-- 
Will