Congress has unlimited power when it comes to all things involving money, they and they alone have the constitutional power of the purse.
If they can void 200+ years of contract law to bail out GM and screw the bond holders they can cut the benefits of federal workers.
Congress had no contract with GM bond holders and did not interfere with any contract involving them.
There are several issues which would stop Congress from abrogating the contract it has with fed workers. Any such law would appear to be a violation of the Constitutional ban on ex post facto Laws.
Such a law would also be a form of confiscation of private property without compensation since the labor agreed to be provided was performed under an agreement as to compensation.
Marshall dealt with similar issues (Gibbons vs Ogden, Hunters Leasee, Yazoo) as long ago as 200 yrs. I have no doubt the courts would rule very quickly to stop any ex post facto changes in fed compensation.