Direct attached remote LAN with VPN failover

[Andy_]

I'm in the situation of connecting a remote LAN via fibre to the local LAN, routed through a pfSense router (carp pair). In case the fibre connection breaks, the router should automatically fail over to a VPN connection, using the Internet as backup connection.

If the backup connection was delivered via another box that has an ip address, this would be a standard situation: gateway group with tier1 to the other LAN's router, tier2 to the backup box. But I fail to find a way to accomplish this with with the very same pfSense box on either side. Using IPSEC there's no gateway to ping and route, using OpenVPN I can't assign a static IP and an upstream gateway AFAICS.
Did I miss something or do I have to install a dedicated VLAN box on both sides?

Regards
Andreas