Saying Amein to Birkas Kohanim

Rabbi Shragga Kallus delivered an enthralling and engaging lecture filled with Torah riddles. One such riddle was: “What are two scenarios where a person would answer ‘amein’ in the middle of his private Shemoneh Esrei?” Typically, all responses are forbidden then. He offered two scenarios. One is relatively rare (still valid as a riddle answer!). The second one is controversial.
One possible scenario is when someone is listening to the chazzan and being Yotzei with the chazzan’s recital of Shemoneh Esrei in lieu of his own. Since he is fulfilling his obligation of Shemoneh Esrei by listening to the chazzan, he must answer amein to each bracha. For various reasons, this technique is seldom employed.
The second scenario Rabbi Kallus offered, while being more common, is not universally accepted.
The Gemara (Sotah 38) says that a kohen has a positive mitzvah to bless Klal Yisrael. The Gemara does not directly discuss whether the person receiving the bracha has any mitzvah. The Sefer Chareidim writes that someone who pays attention to Birkas Kohanim and intends to be included in the blessing is also part of the mitzvah.