I need more information. What application/protocol is used for ROs? Is it IP, FCIP, and so on? Are the layer 3 addressing (e.g. IP) for MO and DR the same or different?
Assume that the layer 3 addressing for MO and DR are different. I think that you need to adjust the IP routing table on the RO router/PIX; so that the IP routing to the MO is more preferrable to the IP routes to the backup DR. The interesting question here is how the applications switch to the new IP address.