Well, I don't think it's costless, simply because I think they are reimbursing the mortgage servicers money for each of these millions of loans in foreberance and the extra costs they incur in the higher serving cost ($500/ln?), plus if the MBS holders still get paid as if borrower/mortgagor is current, don't the GSES' still have to shell out the missed payments now, and get it back at some unknown time in the future?
In the meantime, do the deferred loans accrue interest?