multi-homed nic and nat

Within a government statewide network.
They provide and manage Cisco router. From us it has a 10.x.x.1 address
and next hop is hopefully BorderManager at 10.x.x.2 . Then bind a second
IP address 203.x.x.x and 3 more in the range as secondary IPs.
When BM routes from 10.x.x.2 to 203.x.x.x NAT fails. All of these
addresses I have specified as Public. If I go direct to 203.x.x.x then
NAT works.
Is it possible for this arrangement to work? This is the setup they
recommend for ISA.

Another option I suppose would be to proxy internal mail and web servers.

Regards Warren