I figured it out -
I created two EEM FlexConfig objects:
My_EEM-FTD-MgmtIf_1
event manager applet NAT-FTD-MgmtIf1
event syslog id 622001
action 1 cli command "no nat (mgmt-ftd,outside) source static Host-FTDMgmtIf interface destination static HostFMC_outside HostFMC_outside service tcp_8305 tcp_8305"
action 2 cli command "clear conn address X.X.X.X"
output none
My_EEM-FTD-MgmtIf_2
event manager applet NAT-FTD-MgmtIf2
event syslog id 622001 occurs 2
action 1 cli command "nat (mgmt-ftd,outside) 1 source static Host-FTDMgmtIf interface destination static HostFMC_outside HostFMC_outside service tcp_8305 tcp_8305"
action 2 cli command "clear conn address X.X.X.X"
output none
My_EEM-FTD-MgmtIf_1 removes NAT statement to outside when outside is down (first occurrence of Syslog 622001). It also clears Mgmt interface connections to FMC (IP address X.X.X.X)
My_EEM-FTD-MgmtIf_2 adds NAT statement to outside in position 1 when outside2 is down ("every other" occurrence of Syslog 622001). It also clears Mgmt interface connections to FMC (IP address X.X.X.X)
Apply both FlexConfig objects to FTD using Append. Whenever the outside interface goes down/route to outside is removed from the routing table using route tracking, the static NAT rule for it is removed from the NAT table (this avoids NAT Divert) and when the outside interfaces is up/route tracking is up, the static NAT rule for it is re-added to the NAT table in position 1. Clearing the connections to the FMC is very important because if you don't they hang around and you'll end up in a situation where network traffic goes via ISP1 but FTD management traffic goes to FMC via ISP2.
I hope this helps someone.
Steve
