I think it's also a reference to John the Baptist, to whom Joe compares himself in Act I: "My work was over/I was feeling no pain/Locked up like John the Baptist." Later, in Act II Norma also compares him to John th Baptist: "When he scorned me I/Knew he'd have to die/Let me kiss his severed head."
Salve, Regina, Mater misericordiae
Vita, dulcedo, et spes nostra
Salve, Salve Regina
Ad te clamamus exsules filii Eva
Ad te suspiramus, gementes et flentes
O clemens O pia