I posted a few lines on forwarding address in LSA5 and did not get time to simulate the lab and pass on the results. But packet life has posted a awesome explanantion for the same. Security should be kept in mind while designing and implementing such type of solutions.
Most of time providers never go with these type solutions but in extreme case it could happen only that time if you are owner of both service providers or you are running multiple AS numbers in your SP MPLS/IP VPN cloud.