natd problem -- pass specific IP to internal machine

my natd has been translating fine using:

interface xl0
use_sockets yes
same_ports yes
unregistered_only yes

However, I am having an issue with a particular internal system (solar inverter)
and I would like to be able to tcpdump it on the external interface.

For testing, I tried assigning one of my external IPs to a workstation,
but the workstation doesn't appear to communicate on the external network.
A tcpdump shows syn packets going out on the external address, but none coming
back.  I see an unanswered ARP request for the alias_address from the dslmodem,
which makes sense in that the internal workstation is not directly connected
to that network.
The changes I made:

natd.conf on gateway:

#interface xl0
redirect address 192.168.x.y aa.bb.cc.ee
alias_address aa.bb.cc.dd                  aa.bb.cc.dd is xl0 addr on gateway

internal workstation:

ifconfig re0 alias aa.bb.cc.ee netmask 255.255.255.255

network topology:
   =====dslmodem=== xl0-gateway-fxp0 ===internal=== re0-workstation

What am I missing?

Thanks,

Gary