It dawned on me that maybe the names of the stand-ins were listed in one of the PomPress books, and sure enough both versions of the DS Almanac list: "Alfred Dillay: (2) Stand-in for Barnabas Collins (1967)".
Now, I'm assunming that is possibly referring to episodes like #232 and #233, in which we only see Barnabas' silhouette, or episode #348, in which we only see the aged Barnabas from the back. But perhaps he also filled in for Frid/Barn during rehearsals...