wouldn’t it be a dhcp server functionality rather than fog
FOG could do it with some programming, but its probably easier to do it on the dhcp server side. Its not super simple to do, but basically you would setup dhcp reservations or uniqueness based on the mac address of the target computer. When the dhcp server sees a known mac address then it sends out the alternate pxe boot loader. You would make the default pxe boot loader (like undionly.kpxe) be the default and then override it with reservation settings for the troubled computers. It can be done.
I find its strange that the undi driver doesn’t work on some computers but does on others. The undi specification has been around for 30 years. Most firmware has had time to work out the bugs.