Can a non-Catholic Gospel singer sing at Mass?

In the Church’s Directory for the Application of Principles and Norms on Ecumenism, this issue isn’t directly addressed. But based on the Directory’s provisions for non-Catholic readers at Mass, we think an inference can be made for a non-Catholic cantor or other singer:

The reading of Scripture during a Eucharistic celebration in the Catholic Church is to be done by members of that Church. On exceptional occasions and for a just cause, the Bishop of the diocese may permit a member of another Church or ecclesial Community to take on the task of reader (no. 133).

For similar reasons, including ecumenical ones, a bishop may allow—including through power deputed to his parish priests—a non-Catholic’s singing in such circumstances.