Administration officials are chuckling over what they view as a transparent effort by Federal Reserve Chairman Alan Greenspan to avoid the heat for a politically unpopular Fed stand. The central bank opposes a Clinton effort to consolidate bank supervision, but Greenspan is remaining publicly mum on the issue. Instead, he sent Vice-Chairman David W. Mullins Jr. out to blast the plan. Clintonites speculate that Mullins, who stands virtually no chance of staying on when his term ends in 1995, has nothing to lose. But while Greenspan will likely be replaced in 1996, he dreams of reappointment.