What routing protocols are you using? Typically MPLS providers support BGP. if this is the case I would peer EIGRP with your Asa and inject a default route. You can then inject all private routes plus a default over your MPLS cloud via bgp. I would then redistribute bgp to EIGRP on your MPLS gateway router and use the bgp backdoor 0.0.0.0 command. The ASA and mpls router should peer eigrp. This will lower the administrative distance to 190 from the ebgp learned default route on the MPLS
That way under normal conditions you will have a default route to your ASA and all private routes to your MPLS router. If the asa goes down the default route with 190 via bgp will be injected to the routing table. And everything will flow over the MPLS. If the MPLS goes down you will simply have a default route to your ASA and no private routes. If you build a IPSEC tunnel for backup, interesting traffic will hit the ASA and establish a VPN to the head end office. So long as it is configured properly.
HTH
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