Don't forget his later return in Pennies From Heaven.
He did a small amount of opera, including the title role in Massenet's Le Jongleur de Notre Dame. which at the time he did it was remembered primarily in the version that Massenet revised so that Mary Garden could sing the lead as a trousers role. In 1961, Rall sang two performances of it in Boston.
The previous summer he sang single performances as Turiddu and Don José in Berkshire Music Center student productions at Tanglewood. In 1967, it was announced that he would be singing Cavaradossi and Alwa for Sarah Caldwell’s touring company, American National Opera, but I’m not sure he ended up doing it. Those are the only opera credits I can find for him, but there may have been others. Of course, he was on Broadway singing and dancing in Cry for Us All in 1970 so his opera career was not after his Broadway work. His Tanglewood appearances and that Jongleur were before Milk and Honey and the particularly ill-fated Cafe Crown.
Among his regional credits was the Prince in Rodgers and Hammerstein's Cinderella, also in 1961.