I realise this is a very old post and I am sure that you have already solved the issue, but whilst researching a similar question I came across your entry, and the article I had just been reading seemed to contain the solution - thought I'd reply just in case someone else ended up here. Would this article not get you where you want to be? https://meraki.cisco.com/blog/2014/02/automatic-mpls-to-vpn-failover-now-in-every-mx/ See https://documentation.meraki.com/MX/Deployment_Guides/MPLS_Failover_to_Meraki_Auto_VPN for the actual configuration options. The hostingcentre and site03 MX's use static routes to send traffic over the MPLS - BUT, the "Active" field is set to something other than "Always" thus removing the route and failing back to the auto VPN if the MPLS goes down.
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