Failover based on layer 1

Just a thought here, but I'm assuming you're talking about a system-level script that periodically polls interface status? 'tmsh list net interface', or something like that? Otherwise, the management-level options would all be layer 3 or above. For example, you could set a VLAN failsafe to monitor connectivity to a specific VLAN. If you're truly worried about providing interface redundancy, I would look at trunking the interfaces.