On the Archdiocese of Spokane’s website, Bishop Cupich seems to say that priests can participate:
“As for the specific question of the priests’ participation in the 40 Days for Life vigils, the Bishop recognizes that a given priest in good conscience may feel the need to participate in the vigils and he should never be forced to go against a good and informed conscience. The Bishop only asked that all priests prayerfully reflect on what he has told them, commit themselves to making teaching effectively their first priority and keep in mind the irreplaceable power of the witness of their unity with each other.”
dioceseofspokane.org/news_detail.php?id=147