A solution

As suggested on Hoocoodanode:
National Mortgage loss recognition act of 2011:

All current, non-modified mortgages originated in 2006 or 2007 owned by a privately issued MBS shall be written down by 5% of principal value or $20,000, whichever is less.
Mortgage owners, or servicers as their agents, may request an appraisal of the property, prior to the writedown. If the appraised value of the property is greater than the principal value, no writedown need occur. If the appraised value is less than the principal, the principal shall be written down to the greater of the appraised value or 95% of the remaining principal.

This would speed up the process of clearing out the overprice mortgage loans. By reducing the principal value of the mortgage, the MBS would gain transparency and thus become more liquid. It will also bring some homeowners into positive equity, meaning they are free to sell the house or possibly refinance. Even if they don't have positive equity, the amortization of the loan would be accelerated by the writedown and they will reach positive equity much sooner.