got any idea how to do this in MT? anyone?if the FO would allow your customer subnet through their source-filters (or if they are too ignorant to have any), you should be able to do this without any active routing protocols. Then you would just need to do default route one way and let the returning traffic come the other way.
If you NAT your customers then this could prove more tricky but not impossible. The key is the FO's source filters.