Issuing lease from a secondary pool

[Norman Elton]

Let's say I've got a router (192.168.1.1/24) that is proxying DHCP
requests back to a central server. That scope is starting to fill up,
so I add a "secondary IP" to the router, 192.168.12.1/24. So now it's
handling both the 192.168.1.0/24 network and the 192.168.12.0/24
network in the same layer-2 broadcast domain.

When a client broadcasts for an IP address, the router forwards the
request back to the server, which sees the request coming from
192.168.1.1 and tries to find a lease in the 192.168.1.0/24 pool. But
that pool is now exhausted. I'd like to be able to configure the DHCP
server to issue leases in a secondary pool (in this case,
192.168.12.0/24). As far as I know, the ISC DHCP server will only look
for available leases in the pool that the relaying router is a member
of. Can I configure it to trust that a secondary IP address exists for
a particular relaying router, and to service requests from this
secondary pool?

Any thoughts? Thanks,

Norman