I don't think it'll ever be possible to prove with DNA evidence that anyone was wrongfully executed because the case is closed out once the guy is executed for the crime. If DNA evidence didn't surface in time to save the poor bugger on the day of his execution, nobody's gonna bother digging it up now. Won't save his life anyway, and the point that DNA could have made in his case has already been made in others. A later confession by someone else isn't proof of anything - - some of the guys wrongfully convicted were convicted because they themselves had falsely confessed.
The whole point of DNA testing is that juries can and do make big mistakes and that appeal courts don't always catch them. Thus, of those executed prior to the widespread use of DNA testing, some must inevitably have been innocent.
For a wrongfully EXECUTED person who was truly innocent in all respects, there is the Timothy Evans case, which effectively led to the abolition of capital punishment in the U.K. Evans was an unfortunate individual with a very low IQ who happened to room in the same house as John Christie, a notorious serial killer whom nobody ever suspected at the time. Christie befriended Evans and his wife, killed the wife, advised the dim-witted Evans that he would naturally be suspected by the police, "counselled" Evans on how he could get out of this mess by lying to the police, testified at Evans' trial, got him convicted and hanged. Years later, when Christie was finally exposed as a serial killer, it became obvious that the wrong man had been executed for the murder of Mrs. Evans. The end result was the abolition of capital punishment.