Did you see Milk already? If you haven't yet, do so -- there's also a nice bonus for all opera lovers, a special cameo: Callas's voice. The Tosca reference has been added by the film makers not simply for its metaphorical value, but for a very real historical reason:
"On November 25, 1978, I found myself sitting in the dress circle of San Francisco’s War Memorial Opera House. The opera was Puccini’s “Tosca,” with sets by Jean-Pierre Ponnelle, a designer who at that time was widely admired despite his somewhat innovative approach to the classics. The diva of the evening was the now (and then) legendary Magda Olivero, who without the help of supertitles was able to keep the audience totally riveted. The ovation that followed her performance was the longest I have ever witnessed in the performing arts of any genre. The management brought down the golden curtain, turned on the house lights – and still nobody would go home.
At some point, however – and it’s difficult to remember exactly when – attention in the house turned toward the box of San Francisco Mayor George Moscone. Sitting there was not Moscone but the famous Brazilian soprano Bidu Sayao. Her companion was San Francisco City Supervisor Harvey Milk.
More about Milk and Tosca here, with personal memories from eyewitnesses of that horrible 1978 day:
A long time chorister with the San Francisco Opera remembers that Tosca performance vividly. Tom Reed recalls in detail how those backstage and on stage felt that night as they performed in the Tosca production Milk attended just before he died. Further, long time super Andrew Korniej comments on Tosca's role in Milk.