So I am also interested in this as I have a HA firewall and can only do CARP on the LAN networks. My provider, AT&T, gives me the option of PASS-THROUGH providing "real" WAN IP via DHCP and I lock it down to a single MAC on the Router/Gateway (RG).
So my primary firewall has a spoofed MAC on the WAN that matches the one the RG has configured to hand out leases. My standby HA firewall has the hardware MAC on the WAN interface. The primary gets the "real" WAN IP, publicly routable, and the secondary firewall gets a 192.168.5.X IP from the RG. If I spoofed the MAC on the secondary WAN and shutdown the primary then released/renewed on the secondary it would get the "real" IP on the secondary.
Now I say it is "real" since AT&T does some type of bridge NAT but the NAT table on the RG is still in play.
I am interested in what @chansiuming was looking to do based on my ISP quirks.
I could write a simple script to check CARP status and when it becomes MASTER do the down of WAN, spoof MAC, bring up WAN and boom it should work.