I am pretty sure that a Writ of Pardon does not need to be published or publicized. Not 100% on that though.
This situation has never happened before (obviously). In theory, every action that BHO took while in office could be invalidated.
However, that would be extremely problematic to actually implement, as it could potentially throw out every law he signed, including budgets, etc., where the money has already been spent.
It is likely that Congress and/or SCOTUS would have to figure out how to deal with it.