Being a lector in a Roman Catholic Church
I was asked to be a lector in a Catholic Church for my father-in-law's funeral. I was honored and really wanted to say yes, but decided that I didn't feel comfortable. I was a little surprised that they would allow me to do that. What does Scripture say on the subject? If the roles were reversed, I can't imagine a WELS church would allow someone from a Catholic Church to do a reading during service.
You made the right decision. While we can certainly attend the worship services of churches with which we are not in fellowship, we will refrain from joining in worship (Romans 16:17-18). If we refrain from worshiping in such a setting, we will also not take a leadership role in a service by serving as a lector.
You are correct—our churches follow biblical fellowship principles and would not allow individuals outside our fellowship to take a leadership role in one of our worship services.
When we are united in faith with other Christians, we can express that fellowship by worshiping together and working “together for the truth” (3 John 8). When we are not united in faith with others who profess the Christian faith, we will recognize that they can be part of the Holy Christian Church, but their acceptance of doctrinal errors will lead us to refrain from doing those things that we would normally do with people with whom we are one in faith.