$700 Billion is a pretty big number. Is this the current pool of "squaters" who aren't paying their mortgages? Is this the "shadow" inventory that that people are saying banks are keeping off the market and the houses banks won't foreclose on?
Doing a quick back of the envelope calculation, $700B divided by the average $200K house = 3.5 million houses with unpaid principal balances.
This is only with the three GSE. How many more similar mortgages are sitting at the big banks?