Is there research on opera's topical or current events subtext?

The current Metropolitan Opera production (Feb. 2017) is Antonin Dvorak's Rusalka.  This opera contains what may be viewed as current events references. 

The Grove Dictionary of Music entry on rhetoric and music states that after the Baroque period, the rhetorical underpinnings of music were no longer studied.  But in the 20th C., such highlighted rhetoric in art music is causing a revival of interest.

QUOTE

What still remains to be fully explained is how these critical interrelationships often controlled the craft of composition. These developments are unclear partly because modern musicians and scholars are untrained in the rhetorical disciplines, which since the beginning of the 19th century have largely disappeared from most educational and philosophical system. It was only in the early 20th century that music historians rediscovered the importance of rhetoric as the basis of aesthetic and theoretical concepts in earlier music. An entire discipline that had once been the common property of every educated man has had to be rediscovered and reconstructed during the intervening decades, and only now is it beginning to be understood how much Western art music has depended on rhetorical concepts. ("Rhetoric and Music." Grove Dictionary of Music. Blake Wilson et al.)

UNQUOTE

Does anyone know of research that specifically analyzes the rhetorical aspects of modern era opera since Wagner to now?

Given that directors have much leeway in creating subtexts in classic and contemporary works, this is a topic that is open for more study.