FlatOut 2 LAN multiplayer broadcasts UDP packets on the NIC with the lowest metric. (or at least the discovery part)
So all you got to do is set the "Hamachi" interface's metric to a small value, like 1.
Network Adapters->Hamachi->Properties->ipv4->Properties->Advanced->Metric
(And of course don't forget to take care of the firewall.)
(Also don't forget to change it back when you are finished playing.)