IP routing with BT Business Hub (BT2700HGV)

We have a BT Business Hub (the 2Wire BT2700HGV model), and we have 5
static IPs with our account.

Only one machine is connected to the hub - this is a dedicated firewall
(running IPCop 1.4.10 for those who know about these things). All other
connections on our LAN are made to the firewall. We use the system like
this because the dedicated firewall has more features than the firewall
built in to the BT Hub. The Hub's 'firewall' is configured to allow all
access the the firewall PC.

The Hub is configured to assign one of our static IPs to the dedicated
firewall PC, which works. However, the IPCop firewall software allows
multiple IPs to be set on the same interface (the IPs are set within
IPcop, it does not use DHCP). The problem is that there seems to be no
way to route multiple IPs to the same machine within the Local Network
settings in the BT Hub's software.

I was wondering if anyone had found a way around this? BT support are
reasonably sure it can't be done. Any ideas?

David