My first thought was that Congress could offer immunity from prosecution, but as I think about it, that would likely have to go through Eric Holder, and he is by far the most corrupt and unethical AG we have ever had.
My second was that Congress could grant pardons, but only the President can do that.
It would seem that some law should be on the books which protects people who expose wrong-doing. If there is a law, we need to enforce it–by arresting Eric Holder if necessary for contempt of Congress; and if there is not, we need to create a law. Obama used the word transparency often, presumably because it tested well in focus groups, and because he has never felt the slightest need to make his rhetoric and actions congruent. He was lying, of course, but even so it will be difficult for him to oppose such a law, particularly given the revelations of the past six months, and the coming midterm elections.
Food for thought.
I will add, that in the longer term amending the Constitution to grant Congress the right to pardon people would likely help to weaken the power of the Presidency, which in this late hour is desperately needed.