Transparency is not completely without value
If do this is of course a major operation that gives a totally new meaning to central-bank cooperation though I am not really sure I would like to be on the European side of the bargain. That said if risk adverse central bankers think that the conditions are serious enough to warrant this, why on earth do they not recommend their respective governments to proceed with much more targeted fiscal support measures that can perhaps be better explained to the taxpayer?
I for one would always prefer my government helping directly the mortgage holders who I can at least identify as the beneficiary, than having it give support through the purchase of some debt collateralized with mortgages, where I won’t have a clue whom they are truly benefiting, and the authorities will have to plead blissful ignorance.