Going off of Halopedia, it seems that Chief might be eligible for it already:
Though during the Covenant War, I doubt ONI would have ever allowed a Spartan to be awarded that medallion, purely for morale and PR purposes. Even after the war, Chief is such an iconic and well known figure that the PR disaster that could come from him being considered a prisoner of war would likely lead ONI to continue preventing him from ever receiving it.
Quote:Master Chief Petty OfficerJohn-117, a SPARTAN-II supersoldier, may be eligible for this medal for two distinct actions. First, during the raid on Camp New Hope on May 1, 2531 when his command, Blue Team, was captured by United Rebel Front forces. His capture and interrogation aboard the Forerunner Dreadnought by Covenant forces in November 2552 may also qualify the Spartan to receive the medal. The Prisoner of War Medallion has yet to be bestowed upon any Spartan.[verification needed]
Likewise, our SPARTAN would be eligible for this award, having been captured by Storm Covenant forces on Requiem and held (albeit briefly). before escaping and organizing a breakout of UNSC POWs. (All from SPARTAN Ops in Halo 4).
I agree, though, that it is unlikely the award would ever be presented to a SPARTAN operator. More likely, the medal would be awarded administratively for records, since said records would be classified in any case.