Thanks for the reply Daniel.
Let me propose some follow-up then:
This same loan may be refinanced - if that happens, that's probably good for the owner of the MBS since current rates are *higher* and they'd receive the full balance of the loan. Assuming the borrower is in a position to pay off the mortgage or stick to the payment schedule, there is some place in between those situations that offers mutual benefit.
Perhaps the complexity of the MBS's themselves precludes this singular "negotiation" or perhaps the juice isn't worth the squeeze because there just aren't enough of these borrowers. I know a handful of people who are in this precise spot, most with 500k-1M loans that they could write a personal check for.
It was a fun theory that just doesn't hold up in the real world I guess =)
*edit to add: the discounts priced into the MBS would have been at issuance of the MBS itself right? So, marking to market, the present value has degraded and I would be offering some concrete improvement on the present value of _my_ mortgage.