Its one of the reasons why many use their own hosted FusionHubs or ask other hosting partners to help by providing SpeedFusion Hosting as a service (we do this at Venn globally).
However, you can use latency cut off as a way to achieve what you want. Put all WANs in the SFC tunnel, then set the latency cut off on all WANs.On the ones you want to use in the tunnel set them at a sensible level, on the WANs you don’t want to use set the cut off lower than they can achieve. This affectively removes them from active use in the tunnel - whilst keeping them available to be used in the tunnel.
When the primary WANs fail (or their latency rises above the cut off values set), since all active WAN latencies will be over the cut off value, the other WANs will be used. And you’ll have hot failover that reacts as fast as the latency measurement.