How about something much more simple as a solution?
Just have two FOG servers.
One will be your primary server and production server. The other will just be blank and always on.
When the primary goes down, quickly change your DHCP option 066 to point to the secondary. Problem solved. Fix your FOG server, then change it back.
You could even configure the secondary with all your images and hosts and such if you wanted… but that’s beyond the scope of this conversation (But don’t ever expect domain joining to work on the secondary).