"Yashar Koach" to Cohanim

Q: Is it a proper minhag for everyone to shake the Cohanim's hands and say: "Yashar Koach" after Birkat Cohanim? After all, they are obligated from the Torah to recite the blessing and they also cannot hear the prayers that are now being said.
A: 1. Regarding saying "Yashar Koach" even though the Cohanim are obligated in the mitzvah, there is a Mishnah in Shevi'it (4:2) which says that the poor people who are entitled to take the ownerless crops during the Shemita year would nevertheless go to the owner of the field and say "Thank you", even though the owner was obligated to allow the poor to take the crop from his field during that year. There are versions of the Mishnah which say that everyone agrees that this is permissible. The Rashash (ibid.) said that this Mishnah is the source for the minhag to say "Yashar Koach" to the Cohanim after Birkat Cohanim. He emphasized that even though the Cohain would be neglecting the fulfillment of a positive mitzvah from the Torah if he did not recite the blessing, we learn that the circumstances which cause the giver to provide others with something does not diminish the receivers obligation to express gratitude. Ha-Rav Ovadiah Yosef in Yalkut Yosef (Tefila p. 338) also has written that it is proper to say "Chazak U-Varuch" or "Yashar Koach" based on this idea. Regarding the problem of missing out on the prayers during that time, this is improper. One should therefore say "Yashar Koach" quietly in order not to disturb others or wait until after the prayers so that it is not at the expense of the prayers.