I hope I understand you correctly:
"when mpls is disconnected" = "down".
For the remote site use SRST.
For calls central site->remote site there is a feature in 6.0 cfur/"call forward unregistered".
I don't see any elegant solution for you with 5.1 :-(
AAR is only good if the WAN is full, not for outage situations.
Hope that helps somehow.
/Martin